Exposes an opaque handle for uniformly (cppgc and V8) referring to an
instance of a heap.

Exposes a set of raw write barriers for advances embedders through
subtle::HeapConsistency which is a mirror into write barrier internals.
The following barriers are exposed:
- DijkstraWriteBarrier: Regular Dijkstra-style write barrier (add to
  wavefront);
- DijkstraWriteBarrierRange: Same as DijkstraWriteBarrier but
  operating on a range of slots that are composite (inlined) objects;
- SteeleWriteBarrier: Regular Steele-style write barrier (retreating
  wavefront);

Change-Id: Ib5ac280204686bf887690f72df1cdb506ea6ef70
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2554601Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71381}