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
http://codereview.appspot.com/5607055

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old