This CL moves the multiplication (and squaring) code into natmul.go,
in preparation for cleaning up Karatsuba and then adding Toom-Cook
and FFT-based multiplication.
Change-Id: I7f84328284cc4e1ca4da0ebb9f666a5535e8d7f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/652055
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>