As the next step in supporting huge typed arrays in V8, this moves the
external/data pointer from the FixedTypedArrayBase backing store to the
JSTypedArray instance itself, and replaces the special backing stores
with a plain ByteArray (removing all the code for the FixedTypedArrayBase
class hierarchy). By doing so, we can drastically simplify the system
around typed arrays.

Note: Several places in the code base used to check the instance type
of the elements backing store of a JSTypedArray instead of checking the
elements kind on the JSTypedArray map directly. Those had to be fixed,
since the backing store is now always a ByteArray.

Drive-by-fix: Move all the typed elements access related code into the
elements.cc file to properly encapsulate the accesses.

Doc: http://doc/1Z-wM2qwvAuxH46e9ivtkYvKzzwYZg8ymm0x0wJaomow
Bug: chromium:951196, chromium:965583, v8:4153, v8:7881, v8:9183
Change-Id: I8cc06b190c53e34155000b4560f5f3ef40621646
Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627535
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61855}