Implement WriterTo / ReaderFrom

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.