On my 64-bit machine, despite being 32-bit code, fixed-base
multiplications are 7.1x faster and arbitary multiplications are 2.6x
faster.

It is difficult to review this change. However, the code is essentially
the same as code that has been open-sourced in Chromium. There it has
been successfully performing P-256 operations for several months on
many machines so the arithmetic of the code should be sound.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/10551044