This change moves external pointers into a separate table and turns
external pointers in heap objects into indices into that table.

This CL implements one of two possible ownership models for the table
entries. With this one, every heap object owns its table entries, and
they are allocated when the owning object is allocated. As such, setting
external pointer fields does not require allocation of table entries. On
the other hand, table indices cannot be shared between multiple objects.

This CL does not yet implement freeing of external pointer table
entires. This will later happen by a table garbage collector.

Bug: v8:10391
Change-Id: I4d37785295c25a7d1dcbc9871dd5887b9d788a4f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235700Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Samuel Groß <saelo@google.com>
Cr-Commit-Position: refs/heads/master@{#70204}