[bigint] Karatsuba multiplication

The Karatsuba algorithm is used for BigInts with 34 or more internal
digits, and thanks to better asymptotic complexity provides greater
speedups the bigger the inputs.

Bug: v8:11515
Change-Id: I5ab0e318173ea4a02ced3f156d3c17e0259c5036
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2782283


Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74916}