This CL templatizes some methods in BigInt on the Isolate type, to allow
BigInts to be allocated off-thread from a BigInt literal.

A necessary side-effect is exporting the Isolate class in its entirety,
to allow it to be used as a specializing type for ' HandleFor' in
unittests.

Bug: chromium:1011762
Change-Id: I768f9e4d46a4532d6818d9a67c13801bc5952e5d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2036079
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66105}