Extract libp2p-identity crate

[thomaseizinger]

This issue is part of #3271.

• These modules are described in a single spec: https://github.com/libp2p/specs/blob/master/peer-ids/peer-ids.md
• It will be useful to improve the multiaddr crate: Making the /p2p protocol type-safe multiformats/rust-multiaddr#73
• This crate will be a dependency of pretty much the entire ecosystem. It should be heavily feature-flagged and its API should try and avoid to leak any dependencies.
• We can consider moving PeerRecord and SignedEnvelope into this crate as well or extract them into their own. I am undecided here.

[thomaseizinger]

• We can consider moving PeerRecord and SignedEnvelope into this crate as well or extract them into their own. I am undecided here.

Already running into the first issue as I am starting to work on this.

A PeerRecord depends on Multiaddr as well as PeerId. If we want multiaddr to depend on libp2p-identity, we can't define PeerRecord within libp2p-identity otherwise we have a circular dependency!

[thomaseizinger]

• We can consider moving PeerRecord and SignedEnvelope into this crate as well or extract them into their own. I am undecided here.

Already running into the first issue as I am starting to work on this.

A PeerRecord depends on Multiaddr as well as PeerId. If we want multiaddr to depend on libp2p-identity, we can't define PeerRecord within libp2p-identity otherwise we have a circular dependency!

The solution to this turned out to be to not use the proto definition of PeerId in PeerRecord but treat it as opaque bytes instead that we parse through the functions available in libp2p-identity::PeerId.