querystring: improve unescapeBuffer performance

[mscdex]

Results with included benchmark:

 improvement confidence p.value querystring/querystring-unescapebuffer.js n=10000000 input="%20%21%22%23%24%25%26%27%28%29%2A%2B%2C%2D%2E%2F%30%31%32%33%34%35%36%37" 12.86 % *** 5.432023e-29 querystring/querystring-unescapebuffer.js n=10000000 input="there is nothing to unescape here" 18.04 % *** 6.686024e-40 querystring/querystring-unescapebuffer.js n=10000000 input="there%20are%20several%20spaces%20that%20need%20to%20be%20unescaped" 21.62 % *** 1.254710e-42 querystring/querystring-unescapebuffer.js n=10000000 input="there%2Qare%0-fake%escaped values in%%%%this%9Hstring" 9.11 % *** 1.738303e-14 

CI: https://ci.nodejs.org/job/node-test-pull-request/5885/

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

[Fishrock123]

Didn't this like, used to use a mapping table like a year ago? haha

[mscdex]

@Fishrock123 Not that I'm aware of. stringify() still uses a (different) table though.

[mscdex]

Also, to anyone who might ask why I didn't put this conditional in the for conditional and the else block immediately after the end of the for loop, it's because there is actually a 2% reduction in the performance increase for the 'fake escaped values' input. The results for the other inputs seem to be largely unaffected though.

[jasnell]

This will need a backport PR to land on v4. Landed on v6