This CL introduces the following changes to the experimental
implementation of the object start bitmap, that is evaluated as
a mechanism for resolving inner pointers (behind the flag
v8_enable_conservative_stack_scanning):

- Manually iterate through page objects, instead of using the
  PagedSpaceObjectIterator, for performance (avoid calling
  MakeHeapIterable all the time) and to simplify the handling
  of filler objects.
- Clear bits when reusing evacuated pages of the new space.
- Use the cage base to iterate correctly through code objects.
- Introduce a method for verifying the validity of the object
  start bitmap.
- Minor fixes, additional checks and cleanup.

Bug: v8:12851
Change-Id: I245937ffe6f4b53c4c2dcf5126e8836aec4dc79e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3675099Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80869}