path: improve normalization performance

Some results:

                                                                                             confidence improvement accuracy (*)   (**)  (***)
path/normalize-posix.js n=10000000 path='.'                                                        ***      8.62 %       ±1.91% ±2.54% ±3.30%
path/normalize-posix.js n=10000000 path=''                                                         ***      8.71 %       ±2.00% ±2.67% ±3.49%
path/normalize-posix.js n=10000000 path='/../'                                                     ***      7.54 %       ±1.97% ±2.64% ±3.46%
path/normalize-posix.js n=10000000 path='/foo'                                                     ***     16.09 %       ±2.77% ±3.71% ±4.88%
path/normalize-posix.js n=10000000 path='/foo/bar'                                                 ***     11.90 %       ±1.86% ±2.47% ±3.22%
path/normalize-posix.js n=10000000 path='/foo/bar//baz/asdf/quux/..'                               ***      9.30 %       ±1.98% ±2.64% ±3.45%
path/normalize-win32.js n=10000000 path='.'                                                        ***      4.80 %       ±1.29% ±1.73% ±2.28%
path/normalize-win32.js n=10000000 path=''                                                         ***      4.49 %       ±0.94% ±1.26% ±1.66%
path/normalize-win32.js n=10000000 path='C:\\\\..\\\\'                                                      1.48 %       ±1.56% ±2.08% ±2.72%
path/normalize-win32.js n=10000000 path='C:\\\\foo'                                                ***      3.33 %       ±1.61% ±2.14% ±2.80%
path/normalize-win32.js n=10000000 path='C:\\\\foo\\\\bar'                                          **      2.26 %       ±1.39% ±1.86% ±2.42%
path/normalize-win32.js n=10000000 path='C:\\\\foo\\\\bar\\\\\\\\baz\\\\asdf\\\\quux\\\\..'                 0.35 %       ±1.28% ±1.71% ±2.24%

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines