Instead of creating a separate code object per exported function, we
can share the code per signature, and load the function index from the
{WasmExportedFunction} object which is being passed as an argument
anyway.
This greatly reduces instantiation time for modules with a lot of
exports.
As a next step, we could even share the code across instances, or (with
more work) across isolates.

R=mstarzinger@chromium.org

Bug: chromium:860491
Change-Id: I6438065b2de0df59dce980fb52408a0f475144b3
Reviewed-on: https://chromium-review.googlesource.com/1127660
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54299}