- Move StringForwardingTable implementation to own compilation unit.
- Refactoring preparing for layout change (Introduce explicit record
  class to make transition from contiguous Tagged_t fields to a
  heterogeneous record layout easier).
- Replace RootVisitor pattern for transitioning/cleanup during GC with
  callback.
- Minor cleanups.

Bug: v8:12957
Change-Id: Iae343393f470130eac0c54148a1303b67fb95aa4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3845635Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82730}