... in order to exercise the snapshot/ component from mjsunit tests
and fuzzers.

* Since the serializer and deserializer can now be called at any time
instead of only in a tightly controlled environment, several
assumptions (such as an empty execution stack, no microtasks, no
handles) no longer hold and had to be made configurable through
SerializerFlags.

* Root iteration now skips more root categories which were previously
guaranteed to be empty (e.g. the stack, microtask queue, handles).

* The %SerializeDeserializeNow runtime function triggers
serialization, deserialization, and heap verification on the current
isolate and native context.

Support is not yet complete and will be extended in future work. Once
all mjsunit tests successfully run, we can add a new test mode to
stress serialization.

Bug: v8:10416
Change-Id: Ie7ff441a761257dd7f256d0a33e73227850074ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2159495
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67423}