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.
* Use `unsigned-varint` crate.
* Implement `Display` for `Protocol`.
Gives `ToString` for free.
* Use `Cow` in `AddrComponent`.
* Add `AddrComponent::acquire`.
* Document `AddrComponent::acquire`.
