This patch moves a first function, Compare, from src/objects/bigint.cc
to src/bigint/, to blaze the trail. More to follow!

Bug: v8:11515
Change-Id: Id7fa0b40ea852dbed1360f7ab439cb32d0c15762
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2737295Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73511}