math/big: make Rat.Denom() always return a reference

math/big: make Rat.Denom() always return a reference

The documentation says so, but in the case of a normalized
integral Rat, the denominator was a new value. Changed the
internal representation to use an Int to represent the
denominator (with the sign ignored), so a reference to it
can always be returned.

Clarified documentation and added test cases.

Fixes #3521.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6237045