string_decoder: rewrite implementation

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• the commit message follows commit guidelines

Affected core subsystem(s)

• string_decoder

Description of change

This commit provides a rewrite of StringDecoder that both improves performance (for non-single-byte encodings) and understandability.

Additionally, StringDecoder instantiation performance has increased considerably due to inlinability and more efficient encoding name checking.

Benchmark results:

string_decoder/string-decoder.js encoding=utf8 inlen=32 chunk=16 n=2500000: ./node: 2608500 ./node-master: 2411100 ............. 8.19%
string_decoder/string-decoder.js encoding=utf8 inlen=32 chunk=64 n=2500000: ./node: 3812400 ./node-master: 3689900 ............. 3.32%
string_decoder/string-decoder.js encoding=utf8 inlen=32 chunk=256 n=2500000: ./node: 3995900 ./node-master: 3666700 ............ 8.98%
string_decoder/string-decoder.js encoding=utf8 inlen=32 chunk=1024 n=2500000: ./node: 3990400 ./node-master: 3734300 ........... 6.86%
string_decoder/string-decoder.js encoding=utf8 inlen=128 chunk=16 n=2500000: ./node: 657720 ./node-master: 593080 ............. 10.90%
string_decoder/string-decoder.js encoding=utf8 inlen=128 chunk=64 n=2500000: ./node: 1454800 ./node-master: 1356500 ............ 7.25%
string_decoder/string-decoder.js encoding=utf8 inlen=128 chunk=256 n=2500000: ./node: 1821500 ./node-master: 1741000 ........... 4.62%
string_decoder/string-decoder.js encoding=utf8 inlen=128 chunk=1024 n=2500000: ./node: 1819900 ./node-master: 1750200 .......... 3.98%
string_decoder/string-decoder.js encoding=utf8 inlen=1024 chunk=16 n=2500000: ./node: 83475 ./node-master: 75012 .............. 11.28%
string_decoder/string-decoder.js encoding=utf8 inlen=1024 chunk=64 n=2500000: ./node: 175280 ./node-master: 172150 ............. 1.82%
string_decoder/string-decoder.js encoding=utf8 inlen=1024 chunk=256 n=2500000: ./node: 266000 ./node-master: 247300 ............ 7.56%
string_decoder/string-decoder.js encoding=utf8 inlen=1024 chunk=1024 n=2500000: ./node: 260120 ./node-master: 252230 ........... 3.13%
string_decoder/string-decoder.js encoding=utf8 inlen=4096 chunk=16 n=2500000: ./node: 20576 ./node-master: 17684 .............. 16.35%
string_decoder/string-decoder.js encoding=utf8 inlen=4096 chunk=64 n=2500000: ./node: 46176 ./node-master: 43044 ............... 7.27%
string_decoder/string-decoder.js encoding=utf8 inlen=4096 chunk=256 n=2500000: ./node: 66595 ./node-master: 63134 .............. 5.48%
string_decoder/string-decoder.js encoding=utf8 inlen=4096 chunk=1024 n=2500000: ./node: 67692 ./node-master: 64320 ............. 5.24%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=32 chunk=16 n=2500000: ./node: 792560 ./node-master: 376330 ...... 110.60%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=32 chunk=64 n=2500000: ./node: 2776000 ./node-master: 1388800 ..... 99.88%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=32 chunk=256 n=2500000: ./node: 2552900 ./node-master: 1250700 ... 104.12%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=32 chunk=1024 n=2500000: ./node: 2791300 ./node-master: 1263100 .. 120.99%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=128 chunk=16 n=2500000: ./node: 242150 ./node-master: 100900 ..... 139.99%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=128 chunk=64 n=2500000: ./node: 731910 ./node-master: 308100 ..... 137.56%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=128 chunk=256 n=2500000: ./node: 1880800 ./node-master: 1005100 ... 87.14%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=128 chunk=1024 n=2500000: ./node: 2031900 ./node-master: 1067100 .. 90.41%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=1024 chunk=16 n=2500000: ./node: 28221 ./node-master: 13268 ...... 112.70%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=1024 chunk=64 n=2500000: ./node: 110520 ./node-master: 48485 ..... 127.95%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=1024 chunk=256 n=2500000: ./node: 281630 ./node-master: 155010 .... 81.69%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=1024 chunk=1024 n=2500000: ./node: 509790 ./node-master: 329090 ... 54.91%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=4096 chunk=16 n=2500000: ./node: 7454.5 ./node-master: 3540.4 .... 110.56%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=4096 chunk=64 n=2500000: ./node: 24644 ./node-master: 12107 ...... 103.56%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=4096 chunk=256 n=2500000: ./node: 74954 ./node-master: 40305 ...... 85.97%
string_decoder/string-decoder.js encoding=base64-utf8 inlen=4096 chunk=1024 n=2500000: ./node: 125180 ./node-master: 92216 .... 35.75%
string_decoder/string-decoder.js encoding=base64-ascii inlen=32 chunk=16 n=2500000: ./node: 985010 ./node-master: 451520 ..... 118.15%
string_decoder/string-decoder.js encoding=base64-ascii inlen=32 chunk=64 n=2500000: ./node: 2830500 ./node-master: 1421600 .... 99.10%
string_decoder/string-decoder.js encoding=base64-ascii inlen=32 chunk=256 n=2500000: ./node: 2853400 ./node-master: 1380000 .. 106.77%
string_decoder/string-decoder.js encoding=base64-ascii inlen=32 chunk=1024 n=2500000: ./node: 2389400 ./node-master: 1483300 .. 61.08%
string_decoder/string-decoder.js encoding=base64-ascii inlen=128 chunk=16 n=2500000: ./node: 268370 ./node-master: 126350 .... 112.40%
string_decoder/string-decoder.js encoding=base64-ascii inlen=128 chunk=64 n=2500000: ./node: 756360 ./node-master: 448280 ..... 68.73%
string_decoder/string-decoder.js encoding=base64-ascii inlen=128 chunk=256 n=2500000: ./node: 2099500 ./node-master: 1075000 .. 95.29%
string_decoder/string-decoder.js encoding=base64-ascii inlen=128 chunk=1024 n=2500000: ./node: 1978000 ./node-master: 1120100 . 76.59%
string_decoder/string-decoder.js encoding=base64-ascii inlen=1024 chunk=16 n=2500000: ./node: 30721 ./node-master: 14321 ..... 114.52%
string_decoder/string-decoder.js encoding=base64-ascii inlen=1024 chunk=64 n=2500000: ./node: 132060 ./node-master: 64124 .... 105.94%
string_decoder/string-decoder.js encoding=base64-ascii inlen=1024 chunk=256 n=2500000: ./node: 324520 ./node-master: 185200 ... 75.23%
string_decoder/string-decoder.js encoding=base64-ascii inlen=1024 chunk=1024 n=2500000: ./node: 581660 ./node-master: 425450 .. 36.71%
string_decoder/string-decoder.js encoding=base64-ascii inlen=4096 chunk=16 n=2500000: ./node: 9096.6 ./node-master: 3610.9 ... 151.92%
string_decoder/string-decoder.js encoding=base64-ascii inlen=4096 chunk=64 n=2500000: ./node: 30843 ./node-master: 14610 ..... 111.11%
string_decoder/string-decoder.js encoding=base64-ascii inlen=4096 chunk=256 n=2500000: ./node: 90907 ./node-master: 46596 ..... 95.10%
string_decoder/string-decoder.js encoding=base64-ascii inlen=4096 chunk=1024 n=2500000: ./node: 150890 ./node-master: 98642 ... 52.97%
string_decoder/string-decoder.js encoding=utf16le inlen=32 chunk=16 n=2500000: ./node: 3743100 ./node-master: 2683600 ......... 39.48%
string_decoder/string-decoder.js encoding=utf16le inlen=32 chunk=64 n=2500000: ./node: 6516300 ./node-master: 4930200 ......... 32.17%
string_decoder/string-decoder.js encoding=utf16le inlen=32 chunk=256 n=2500000: ./node: 6235800 ./node-master: 5263600 ........ 18.47%
string_decoder/string-decoder.js encoding=utf16le inlen=32 chunk=1024 n=2500000: ./node: 6349900 ./node-master: 4837100 ....... 31.28%
string_decoder/string-decoder.js encoding=utf16le inlen=128 chunk=16 n=2500000: ./node: 973190 ./node-master: 777490 .......... 25.17%
string_decoder/string-decoder.js encoding=utf16le inlen=128 chunk=64 n=2500000: ./node: 3124800 ./node-master: 2180500 ........ 43.31%
string_decoder/string-decoder.js encoding=utf16le inlen=128 chunk=256 n=2500000: ./node: 4903100 ./node-master: 3918900 ....... 25.11%
string_decoder/string-decoder.js encoding=utf16le inlen=128 chunk=1024 n=2500000: ./node: 4909800 ./node-master: 3802800 ...... 29.11%
string_decoder/string-decoder.js encoding=utf16le inlen=1024 chunk=16 n=2500000: ./node: 131990 ./node-master: 95528 .......... 38.17%
string_decoder/string-decoder.js encoding=utf16le inlen=1024 chunk=64 n=2500000: ./node: 401520 ./node-master: 323660 ......... 24.05%
string_decoder/string-decoder.js encoding=utf16le inlen=1024 chunk=256 n=2500000: ./node: 964750 ./node-master: 824650 ........ 16.99%
string_decoder/string-decoder.js encoding=utf16le inlen=1024 chunk=1024 n=2500000: ./node: 1424400 ./node-master: 1378400 ...... 3.33%
string_decoder/string-decoder.js encoding=utf16le inlen=4096 chunk=16 n=2500000: ./node: 33139 ./node-master: 24806 ........... 33.59%
string_decoder/string-decoder.js encoding=utf16le inlen=4096 chunk=64 n=2500000: ./node: 101440 ./node-master: 77741 .......... 30.48%
string_decoder/string-decoder.js encoding=utf16le inlen=4096 chunk=256 n=2500000: ./node: 242900 ./node-master: 203300 ........ 19.48%
string_decoder/string-decoder.js encoding=utf16le inlen=4096 chunk=1024 n=2500000: ./node: 358530 ./node-master: 336270 ........ 6.62%


string_decoder/string-decoder-create.js encoding=ascii n=25000000: ./node: 17123000 ./node-master: 7115100 .. 140.66%
string_decoder/string-decoder-create.js encoding=utf8 n=25000000: ./node: 3912500 ./node-master: 2699900 ..... 44.91%
string_decoder/string-decoder-create.js encoding=utf-8 n=25000000: ./node: 3843400 ./node-master: 2118000 .... 81.46%
string_decoder/string-decoder-create.js encoding=base64 n=25000000: ./node: 3112700 ./node-master: 2427900 ... 28.21%
string_decoder/string-decoder-create.js encoding=ucs2 n=25000000: ./node: 3626400 ./node-master: 2487700 ..... 45.77%
string_decoder/string-decoder-create.js encoding=UTF-8 n=25000000: ./node: 2867000 ./node-master: 1915200 .... 49.70%
string_decoder/string-decoder-create.js encoding=AscII n=25000000: ./node: 7559200 ./node-master: 4634900 .... 63.09%
string_decoder/string-decoder-create.js encoding=UTF-16LE n=25000000: ./node: 2690700 ./node-master: 1747700 . 53.96%