This replaces another use of std::vector. Stack states (consisting of
locals plus operand stack) are also typically small, thus optimize for
those cases.
Using StackVector as part of CacheState requires the definition of move
constructors and copy constructors, plus a few other methods.

R=tebbi@chromium.org

Bug: v8:8423
Change-Id: I5d39c1ebc4d6d65e4849dd06c556114cd2cd36ff
Reviewed-on: https://chromium-review.googlesource.com/c/1380053
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58429}