This CL cleans up a few things as noted by binji in
https://github.com/WebAssembly/spec/pull/979, plus a few more I found
along the way.
In particular:
1) Remove the unused and incorrect {bytesWithHeader} method.
2) Introduce kMaxVarInt32Size and kMaxVarInt64Size constants.
3) Remove redundant {ensure_space} calls (irrelevant for performance).
4) Use {toModule} method instead of duplicating code.
5) Merge two identical leb encoding implementations.

R=titzer@chromium.org
CC=​binji@chromium.org

Change-Id: Idec74e2e46a71766107c182a4176c516d883adad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511273
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60124}