Now that import wrappers are no longer specialized to an index, they
can be cached in the native module, keyed by
(WasmImportCallKind, FunctionSig). This saves instantiation time and
also fixes a (slow) memory leak.

R=mstarzinger@chromium.org

Change-Id: I5197bbfae79d6e811a01289b990db445373eea6c
Reviewed-on: https://chromium-review.googlesource.com/c/1270943
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56526}