Add support for decoding anyref globals, and some drive-by cleanups.

R=titzer@chromium.org

Bug: v8:7581
Change-Id: I3527f996a3fa1a890918a5924e951fbafa394cf2
Reviewed-on: https://chromium-review.googlesource.com/997655Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52391}