This simplifies the double-indirection used for the signature cache in
function imports and also reduces the memory its memory footprint. Also
switch to use the local zone as an underlying storage.

R=rossberg@chromium.org
BUG=v8:6127

Change-Id: I8bc6cf13f2ce9ffa02485e76b7e36f389c9e02e5
Reviewed-on: https://chromium-review.googlesource.com/483443Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44768}