querystring: improve parse() performance

These changes improve parse() performance from ~11-30% on all of
the existing querystring benchmarks.

PR-URL: https://github.com/nodejs/node/pull/4675


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>