Replace deprecated Bytes from crypto.Key

The aforementioned method was deprecated and
removed in `v0.9.0` of `go-libp2p-core`.
The recommended replacement is using the
`crypto.MarshalPublicKey` function. See:
libp2p/go-libp2p-core#204

pkg/net/libp2p/identity.go

@@ -52,7 +52,7 @@ func (i *identity) Marshal() ([]byte, error) {
 		}
 	}
 
-	pubKeyBytes, err := pubKey.Bytes()
+	pubKeyBytes, err := libp2pcrypto.MarshalPublicKey(pubKey)
 	if err != nil {
 		return nil, err
 	}