This changes the encoding of values stored in exception objects from a
typed uint16 array (a JSObject) to a {FixedArray} instead. Note that it
increases the memory footprint of the encoding, but will allow accessing
elements directly from generated code and also encode reference types
properly. The memory footprint can/should be optimized only after the
implementation is feature complete.

R=clemensh@chromium.org
BUG=v8:8341

Change-Id: If67c4e498d815e14f95d014e6a1f7a6725aa0b3a
Reviewed-on: https://chromium-review.googlesource.com/c/1293371
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56854}