The {Vector} class stores the size in a {size_t} since a while, but
many accessors and constructors still accept an {int}. This CL removes
all {int} uses except for the explicit {length()} accessor. It also
adds a comment to avoid this accessor if possible.
The {StrLength} function still has several users outside of vector.h,
which I plan to remove in a follow-up CL.

R=mstarzinger@chromium.org

Bug: v8:8834
Change-Id: I33c5b0e8b8b2cb3531716c1d99e4516a13d6ba1f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1578480Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60950}