This CL doesn't change behavior, only refactors MemoryAllocator:

* De-templatify class, MemoryAllocator is used on slow path and doesn't
  really need templates for performance.
* Rename FreeMode names
* Move methods into private section of class

Change-Id: I7894fba956dcd7aa78ad0284d0924662fef4acae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3379812Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78580}