querystring: improve stringify() performance

[mscdex]

Benchmark results:

                                                                        confidence improvement accuracy (*)   (**)  (***)
 querystring/querystring-stringify.js n=1000000 type='array'                  ***     27.37 %       ±0.86% ±1.15% ±1.50%
 querystring/querystring-stringify.js n=1000000 type='encodelast'             ***     19.40 %       ±1.27% ±1.70% ±2.24%
 querystring/querystring-stringify.js n=1000000 type='encodemany'             ***      9.39 %       ±1.33% ±1.78% ±2.34%
 querystring/querystring-stringify.js n=1000000 type='multiprimitives'        ***     56.64 %       ±1.93% ±2.59% ±3.43%
 querystring/querystring-stringify.js n=1000000 type='noencode'               ***     26.66 %       ±4.15% ±5.59% ±7.41%

Checklist

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

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31666/

[benjamingr]

@bricss why are you upvoting every new PR :D?

[bricss]

@benjamingr I wouldn't consider this as a upvote, but as regular like 🙂

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31701/

[mscdex]

I've made further changes that have netted even larger improvements across the board.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31702/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31852/