- Convert S.p.{trim, trimLeft, trimRight} to TFJ
  - Fast paths for one/two byte strings
  - Added StringTrimAssembler
- Added helper kStringTrim runtime to handle slow paths

Quick measurements show >2.7x improvement:
https://github.com/peterwmwong/v8-perf/tree/master/string-trim

Bug: v8:6680
Change-Id: I79929129aa3d5dea20f094d648afe46adbf61a49
Reviewed-on: https://chromium-review.googlesource.com/647647Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47853}