math/bits: add extended precision Add, Sub, Mul, Div

Port math/big pure go versions of add-with-carry, subtract-with-borrow,
full-width multiply, and full-width divide.

Updates #24813

Change-Id: Ifae5d2f6ee4237137c9dcba931f69c91b80a4b1c
Reviewed-on: https://go-review.googlesource.com/123157


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>