This CL changes IncrementalStringBuilder to write short strings
directly to {current_part_} instead of shortening {current_part},
allocating a new part and concatenate everything using ConsString.

This optimization requires the IncrementalStringBuilder to either
use two byte encoding, or the incoming string is flat with one byte
representation.

This CL improves stack trace serialization micro benchmarks up to 10%.

Bug: v8:8742
Change-Id: I5cc8339be8035c42438381883544d108591fb945
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1647696
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62075}