This CL introduces our own minimal SmallVector implementation and uses
it in several places (more might follow).
I measured that in the majority of cases, these vectors are quite small
(<= 8 elements), so we will avoid any heap allocation in those cases.

R=mstarzinger@chromium.org
CC=titzer@chromium.org

Bug: v8:8423
Change-Id: I93a26b3303a10fe1dc93186430e20333ea4970a8
Reviewed-on: https://chromium-review.googlesource.com/c/1378178
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58323}