Create a high level API for securing messages

[jvz]

Consider the design of Themis: https://docs.cossacklabs.com/themis/crypto-theory/cryptosystems/secure-message/

And consider the NaCl and libsodium API, the box APIs for authenticated public-key encryption, and the sealed box APIs for anonymous public-key encryption.

Create an analogous API here using XChaCha20-Poly1305 and X25519. This should cover sending encrypted messages and signed messages.

Essentially, given Alice and Bob both have XDH keypairs, we can compute a shared secret to generate a symmetric key. Implementing a perfect-forward-secrecy variant involving ephemeral keypairs and signatures for authentication rather than the static XDH keys would be out of scope for messages at this level and is scoped in #6.