Rewrite the reducer in terms of the ordinary keyed-store reducer and
reuse the existing serializer machinery for that as well.

Bug: v8:7790
Change-Id: I5909739feee1d77dca1827166bad3d2a61561784
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1760807Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63254}