Implement WriterTo / ReaderFrom

[FiloSottile]

If we implement io.ReaderFrom on stream.EncryptWriter and io.WriterTo on stream.DecryptReader we can save one(?) copy in io.Copy invocations, because io.Copy can skip the buffer (if the other side didn't already implement WriterTo / ReaderFrom). That would help throughput. See also #646.

We can also use the WriterTo / ReaderFrom on the underlying src / dst. This might or might not combine well because we still need somewhere to do the encryption/decryption.

For the new age.EncryptReader, we'll need to first find a way to wire the io.WriterTo through the io.MultiReader.