In the process:

- add volatile types for FastJSArray and remove the length_fast accessor
  from JSArray with the application of more rigorous typing.
- add micro benchmarks for testing all the interesting slice cases

Also update a few assorted places in .tq code to make them more
idiomatic.

Change-Id: I76ec2bb25b65a869180af1f7288419dc1f0a9c37
Reviewed-on: https://chromium-review.googlesource.com/c/1281603
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56806}