This moves:

- ExternalReferenceEncoder to codegen/external-reference-encoder.h
- SerializerDeserializer to snapshot/serializer-deserializer.h
- Checksum() to snapshot/snapshot-utils.h

serializer-common.h and .cc are removed.

Tbr: clemensb@chromium.org,ulan@chromium.org
Bug: v8:10416
Change-Id: I36a242dcc1ad8833374aa567f73e0d4a75632c58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144118
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67281}