This CL introduces one-letter shorthands to HeapTypes, and fixes
signatures to be in sync with the ValueType and HeapType shorthands.

Bug: v8:7748
Change-Id: I4cc8e26d6523074bc36bf2d29289e63a23e80ddc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2249672
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68425}