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.
* secio: keep ciphertext if start_send is not ready
Otherwise ciphertext may be thrown away and once sent, the cipher states
have diverged, resulting in nonsense messages on decryption side.
* Also attempt to send pending data `Sink::close`.
* Inline `send_pending`.
* Use `unsigned-varint` crate.
* Implement `Display` for `Protocol`.
Gives `ToString` for free.
* Use `Cow` in `AddrComponent`.
* Add `AddrComponent::acquire`.
* Document `AddrComponent::acquire`.
* 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.
