elliptic-curve: derive Copy on PublicKey (#666)

The new trait bounds on `AffineArithmetic` ensure that `AffinePoint` is
always `Copy`. So the previously imposed trait bounds prevent `Copy`
impls which should be available by default.

elliptic-curve/src/public_key.rs

@@ -59,7 +59,7 @@ use alloc::string::{String, ToString};
 /// When the `pem` feature of this crate (or a specific RustCrypto elliptic
 /// curve crate) is enabled, a [`FromStr`] impl is also available.
 #[cfg_attr(docsrs, doc(cfg(feature = "arithmetic")))]
-#[derive(Clone, Debug)]
+#[derive(Copy, Clone, Debug)]
 pub struct PublicKey<C>
 where
     C: Curve + ProjectiveArithmetic,
@@ -260,14 +260,6 @@ where
     }
 }
 
-impl<C> Copy for PublicKey<C>
-where
-    C: Curve + ProjectiveArithmetic,
-
-    AffinePoint<C>: Copy + Clone + Debug,
-{
-}
-
 impl<C> Eq for PublicKey<C>
 where
     C: Curve + ProjectiveArithmetic,