feat: allow serializing handshake state

[Stypox]

• adds feature "serde" which enables serialization for HandshakeState, to e.g. save the state to disk for handshakes that take a long time to complete
• this required making a lot of things serializable across the codebase
• since the Random, Dh, Cipher, Hash, Kem traits are all used dynamically, I used the typetag trait to serialize those
• serde only supports bytestrings up to 32 bytes, so I used serde_bytes for longer ones
• for common hash types I made a wrapper that bridges between serde and the crypto libraries' SerializableState
• blake2 and blake3 do not implement SerializableState unfortunately, so in my first commit they are not enabled when the serde feature is passed
• my second commit brings support for serialization for blake2 as well, although the upstream PR is still pending: blake2: implement SerializableState RustCrypto/hashes#906 (so the cargo dependency currently points to my fork)

Thanks for your work on Snow!