math/big: move division into natdiv.go

Code moved and functions reordered to be in a consistent
top-down dependency order, but otherwise unchanged.

First step toward commenting division algorithms.

Change-Id: Ib5e604fb5b2867edff3a228ba4e57b5cb32c4137
Reviewed-on: https://go-review.googlesource.com/c/go/+/321077


Trust: Russ Cox <rsc@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>