math/big: add (*Int).FillBytes

Replaced almost every use of Bytes with FillBytes.

Note that the approved proposal was for

    func (*Int) FillBytes(buf []byte)

while this implements

    func (*Int) FillBytes(buf []byte) []byte

because the latter was far nicer to use in all callsites.

Fixes #35833

Change-Id: Ia912df123e5d79b763845312ea3d9a8051343c0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/230397


Reviewed-by: Robert Griesemer <gri@golang.org>