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.

The original version of this patch had an overly agressive assert that has
been loosened.

TBR=jgruber@chromium.org

Change-Id: I56870862f4b124d1b38372daa326182a526c874c
Reviewed-on: https://chromium-review.googlesource.com/c/1291375Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56829}