querystring: improve performance

[mscdex]

Results:

                                                                   confidence improvement accuracy (*)     (**)    (***)
 querystring/querystring-parse.js n=1000000 type='altspaces'             ***    985.03 %      ±27.88%  ±40.04%  ±58.85%
 querystring/querystring-parse.js n=1000000 type='encodefake'            ***   1253.69 %      ±26.56%  ±38.14%  ±56.07%
 querystring/querystring-parse.js n=1000000 type='encodelast'            ***   1012.30 %      ±26.81%  ±38.49%  ±56.56%
 querystring/querystring-parse.js n=1000000 type='encodemany'            ***    712.02 %      ±34.90%  ±50.11%  ±73.65%
 querystring/querystring-parse.js n=1000000 type='manypairs'             ***      4.42 %       ±1.35%   ±1.85%   ±2.52%
 querystring/querystring-parse.js n=1000000 type='multicharsep'          ***   1053.69 %      ±21.01%  ±30.15%  ±44.28%
 querystring/querystring-parse.js n=1000000 type='multivalue'            ***   1161.95 %      ±41.03%  ±58.93%  ±86.66%
 querystring/querystring-parse.js n=1000000 type='noencode'              ***   1094.45 %      ±96.37% ±138.44% ±203.66%

 querystring/querystring-stringify.js n=1000000 type='array'             ***      8.28 %       ±2.20%   ±2.94%   ±3.84%
 querystring/querystring-stringify.js n=1000000 type='noencode'           **      3.38 %       ±2.43%   ±3.24%   ±4.21%

Checklist

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