t caches the value of z squared, but wasn't getting correctly
initialised when the value of z was reset.
R=golang-dev
CC=golang-dev
https://golang.org/cl/6850091
Package bn256 implements a particular bilinear group at the 128-bit
security level.
R=golang-dev, remyoudompheng, r, r, akumar
CC=golang-dev
https://golang.org/cl/6402052