They both now run fast (due to utilizing transitions instead of always
creating new maps) and sealed or non-extensible objects can stay in
fast mode after transitioning.
This almost entirely reuses the code for transitioning objects
frozen by Object.freeze(), with the added benefit of freeing
up a bit on the map (we no longer keep track of frozen-ness,
as that bit wasn't used for anything interesting).
BUG=v8:3662,chromium:115960
LOG=y
Review URL: https://codereview.chromium.org/776143005
Cr-Commit-Position: refs/heads/master@{#25759}
