Along with BigInt.prototype. Their functions only have skeleton
implementations. The purpose of this change is to make it easier
to gradually increase test coverage (e.g. for toString(radix)).

Of course this is still behind the --harmony-bigint flag.

Bug: v8:6791
Change-Id: Ic307fd9165c56ac782fba18d648ce893daaa718f
Reviewed-on: https://chromium-review.googlesource.com/671209
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48094}