mat/big: add raw access to Int bits

mat/big: add raw access to Int bits

This is a minimal API extension, it makes it possible
to implement missing Int functionality externally w/o
compromising efficiency. It is the hope that this will
reduce the number of feature requests going directly
into the big package.

Also: Fixed some naming inconsistencies: The receiver
is only called z when it is also the result.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/5607055