- Use a StringShape instead of repeatedly querying type.
- Add a shortcut for already-flat strings.
- Unhandlify where possible (all except SlowFlatten).
- Mark String::Flatten and StringShape methods V8_INLINE.
- Add a specialized ConsString::IsFlat overload.

Drive-by: Various (add const, remove this->, helper methods).

Bug: v8:12195
Change-Id: If20df12bc29c29cff2005fdc9bd826ed9f303463
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3259527
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77701}