This factors out a {WasmCodeAllocator} which manages all code
reservations and allocations for a {NativeModule}. This will allow for
better testing of this component (which will be added in a separate CL).

R=titzer@chromium.org

Change-Id: I09727c30afc533e95569276147792d0e641b0507
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605738
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61445}