We already have all the functionality available in the
CodeStubAssembler, so this is merely connecting the dots.

Drive-by-fix: Improve code generation for StringCharCodeAt
to properly mark runtime entries as deferred and just use
a single slow-path.

Bug: v8:5049
Change-Id: I76793c823b23f676e65cdb717558473edb6b91cd
Reviewed-on: https://chromium-review.googlesource.com/577533
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46774}