Decouple `protocol` and `session` levels

[fjarri]

Ideally we would want protocol to know nothing of session. But currently there are several back-references:

• LocalError and RemoteError are used on both levels. Should we have each layer have their own?
• EchoRoundError is declared at the session level, but used at the protocol level as one of the variants of ReceiveError
• protocol/serialization needs to know of WireFormat

Edit: Fixed/invalidated:

• FirstRound::new() needs to know about SessionId (see also SessionId improvements #9) - SessionId improvements #9 is closed by Tie SessionId to SessionParameters::Digest #41,
• Protocol trait uses Digest trait, which is defined at the session level — using RustCrypto traits directly now