querystring: improve parse() performance

Here are the results with existing benchmarks (and newly added input/type) for the changes in this PR:

                                                                  improvement confidence      p.value
 querystring/querystring-parse.js n=5000000 type="encodefake"          7.60 %        *** 5.588287e-48
 querystring/querystring-parse.js n=5000000 type="encodelast"         23.85 %        *** 3.772582e-43
 querystring/querystring-parse.js n=5000000 type="encodemany"         13.75 %        *** 7.027748e-27
 querystring/querystring-parse.js n=5000000 type="manypairs"           7.09 %        *** 2.271675e-49
 querystring/querystring-parse.js n=5000000 type="multicharsep"       19.71 %        *** 6.757215e-33
 querystring/querystring-parse.js n=5000000 type="multivalue"         37.01 %        *** 4.461087e-58
 querystring/querystring-parse.js n=5000000 type="multivaluemany"     33.38 %        *** 4.410104e-59
 querystring/querystring-parse.js n=5000000 type="noencode"           27.66 %        *** 8.234363e-41

CI: https://ci.nodejs.org/job/node-test-pull-request/5932/

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

• querystring