Instead of allocating a fixed array in which we write individually allocate
mutable heap numbers, we can allocate a byte array large enough to cover all
heap numbers. That avoids expensive allocation calls, write barriers, and
temporary memory O(number of heap numbers) (we'll just have 1 byte empty byte
array as overhead).

Change-Id: I4ffe16dd91e97a2e8cd8e36ff5af37375599276b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1613993
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61581}