Sign then Encrypt

Just as a heads up this is wacky as shit because of some weird merging and rebasing…I'm still looking at it but if you see anything odd please let me know.
@El--Presidente I was wrong about the key being non-ephemeral. The key is derived from a freshly null-initialized `ECC` instance, so it is a random oone. The `ECC` instance is discarded after encryption, so the key is ephemeral. No worries, then.