(Sending to r because of the API change.)

This change alters the API for crypto/elliptic to permit different
implementations in the future. This will allow us to add faster,
constant-time implementations of the standard curves without any more
API changes.

As a demonstration, it also adds a constant-time implementation of
P224. Since it's only 32-bit, it's actually only about 40% the speed
of the generic code on a 64-bit system.

R=r, rsc
CC=golang-dev
https://golang.org/cl/5528088