The WASM engine compiles per-import wrappers for callables imported
into a WASM instance that have one of a number of different shapes,
depending on the type of the imported function and whether there is
a signature match. This CL introduces an enum with a value for each
case in preparation for introducing a per-kind cache.

R=mstarzinger@chromium.org
CC=clemensh@chromium.org

Change-Id: If9b7355ff7c57a329c096f93f3624bc3d6c74e3f
Reviewed-on: https://chromium-review.googlesource.com/1243045
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56210}