This CL creates the invariant that the BigInt class treats
BigInt objects as immutable. Writing to new BigInt objects
as part of their construction is done by the MutableBigInt
helper class, which in turn is hidden as an implementation
detail in bigint.cc.
As a side effect, this refactoring enforces right-trimming
checks for all newly created BigInts, and ensures that all
BigInt allocations possibly exceeding kMaxLength check for
this case and throw a RangeError instead of crashing.

Bug: v8:6791
Tbr: mlippautz@chromium.org
Change-Id: Id239746108e6b076b47a03ba37462001eb501507
Reviewed-on: https://chromium-review.googlesource.com/742329
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49462}