This CL changes the way stack trace serialization appends numbers.
Instead of converting plain int to Handle<String>, they are converted
to char*. The reason is that appending a Handle<String> causes the
IncrementalStringBuilder to shrink the current part and create two
ConsString instances to concatenate the accumulator, the current part and
the passed in Handle<String>.

This CL improves stack trace serialization micro benchmarks by ~12%.

Bug: v8:8742
Change-Id: I174667379084381245827cb979f91db3c59ce75e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1643169
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61996}