Add a verify function to PublicKey

To be uniform with `XOnlyPublicKey` add a `verify` function to the
`PublicKey`.

src/key.rs

@@ -696,6 +696,16 @@ impl PublicKey {
             (XOnlyPublicKey(xonly_pk), parity)
         }
     }
+
+    /// Checks that `sig` is a valid ECDSA signature for `msg` using this public key.
+    pub fn verify<C: Verification>(
+        &self,
+        secp: &Secp256k1<C>,
+        msg: &Message,
+        sig: &ecdsa::Signature,
+    ) -> Result<(), Error> {
+        secp.verify_ecdsa(msg, sig, self)
+    }
 }
 
 /// This trait enables interaction with the FFI layer and even though it is part of the public API