reduce libp2p dependencies

the libp2p feature pulls in libp2p entirely:

Line 17 in 44051ef

    
           libp2p = { version = "0.53", features = ["ed25519", "secp256k1"], optional = true }

but only identity and MultiAddr are used

discv5/src/node_info.rs

Lines 7 to 11 in 44051ef

    
           use libp2p::{ 
        
               identity::{KeyType, PublicKey}, 
        
               multiaddr::Protocol, 
        
               Multiaddr, 
        
           };

which are available as standalone crates:

multiaddr and libp2p-identity

Resolved in #255

	use libp2p::{
	identity::{KeyType, PublicKey},
	multiaddr::Protocol,
	Multiaddr,
	};