Dispatch serialization of keys based on key type

- Add some tests to check high-level roundtrip

tests/crypto/secp256k1.py

@@ -0,0 +1,22 @@
+from libp2p.crypto.secp256k1 import create_new_key_pair
+from libp2p.crypto.serialization import deserialize_private_key, deserialize_public_key
+
+
+def test_public_key_serialize_deserialize_round_trip():
+    key_pair = create_new_key_pair()
+    public_key = key_pair.public_key
+
+    public_key_bytes = public_key.serialize()
+    another_public_key = deserialize_public_key(public_key_bytes)
+
+    assert public_key == another_public_key
+
+
+def test_private_key_serialize_deserialize_round_trip():
+    key_pair = create_new_key_pair()
+    private_key = key_pair.private_key
+
+    private_key_bytes = private_key.serialize()
+    another_private_key = deserialize_private_key(private_key_bytes)
+
+    assert private_key == another_private_key