feat: introduce libp2p-identity crate

This patch combines the `libp2p_core::identity` and `libp2p_core::peer_id` modules into a new crate: `libp2p-identity`.

Resolves https://github.com/libp2p/rust-libp2p/issues/3349.

Pull-Request: #3350.

examples/identify/src/main.rs

@@ -38,10 +38,10 @@
 
 use futures::prelude::*;
 use libp2p::{
-    core::{multiaddr::Multiaddr, upgrade::Version, PeerId},
+    core::{multiaddr::Multiaddr, upgrade::Version},
     identify, identity, noise,
     swarm::{Swarm, SwarmEvent},
-    tcp, yamux, Transport,
+    tcp, yamux, PeerId, Transport,
 };
 use std::error::Error;