This change replaces all direct ECDSA/RSA sign and decrypt operations
with calls through the crypto.Signer and crypto.Decrypter interfaces.

This is a follow-up to https://go-review.googlesource.com/#/c/3900/
which added crypto.Decrypter and implemented it for RSA.

Change-Id: Ie0f3928448b285f329efcd3a93ca3fd5e3b3e42d
Reviewed-on: https://go-review.googlesource.com/7804Reviewed-by: Adam Langley <agl@golang.org>