This moves the internal fields on {WasmExportedFunction} objects from
being properties with private symbols to a separate structure instead.
The new {WasmExportedFunctionData} structure can hang off the underlying
shared function info which is created for each exported function. This
reduces the number of transitions, speeds up instantiation, and makes it
easier to reach them from generated code (in the future).

R=titzer@chromium.org
BUG=v8:7424

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Iaa733b6c9f7bea96246d6680756aa7101669a1a9
Reviewed-on: https://chromium-review.googlesource.com/1047025
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53040}