Test tracking issue.

[kayabaNerve]

This issue is meant to track everything left untested.

The main priority is getting test vectors from the Go implementation of double-ratchet. Notably:

• HKDF. These are actually RFC-5869 test vectors, as the above library used Go's standard library impl.
• Encryption/decryption. This should handle the encryption key derivation/signature checks as well. I don't see a need to test those on their own given that they're effectively just HKDF/HMAC calls.
• Ratchet stepping. The key part of the Double Ratchet protocol; the actual double ratchet. As of right now, this library doesn't implement this properly at all.

[kayabaNerve]

HKDF has been moved to libp2p's implementation.

The Enc/Dec test stands.

Ratcheting should be implemented now. I want to add a fuzz test against self, along with a comprehensive vectorized test sourced from the Go library.