Decrypter is an interface to support opaque private keys that perform
decryption operations. This interface is analogous to the crypto.Signer
interface.

This change introduces the crypto.Decrypter interface and implements
the crypto.Decrypter interface for rsa.PrivateKey with both OAEP and
PKCS#1 v1.5 padding modes.

Change-Id: I433f649f84ed3c2148337d735cafd75f1d94a904
Reviewed-on: https://go-review.googlesource.com/3900Reviewed-by: Adam Langley <agl@golang.org>