Refactor multiaddr crate.
- Remove `AddrComponent`. Instead `Protocol` directly contains its
associated data.
- Various smaller changes around conversions to Multiaddr from other
types, e.g. socket addresses.
- Expand tests to include property tests which test encoding/decoding
identity.
* Introduce NULL cipher and allow more configuration.
* Back to using the hash-code for handshake.
Using `Endpoint` would be incompatible with the existing protocol.
* Add comments.
