No asymptotic improvements, and none are planned either.
Minor speedups (25-50%) through reduced overhead: accessing Digits
is faster than working with Handle<BigInt>, and this implementation
avoids allocating intermediate results.

Bug: v8:11515
Change-Id: I2aab2b1c5c9cbb910800161b8514c497daf2b587
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3149453Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76793}