BigInt is a new primitive type of arbitrary precision integers,
proposed in https://tc39.github.io/proposal-bigint.

This CL introduces a corresponding instance type, map, and C++
class to V8 and adds BigInt support to a few operations (see the
test file). Much more is to come. Also, the concrete representation
of BigInts is not yet fixed, currently a BigInt is simply a wrapped
Smi.

Bug: v8:6791
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ia2901948efd7808f17cfc945f0d56e23e8ae0b45
Reviewed-on: https://chromium-review.googlesource.com/657022Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47956}