elliptic-curve: add explicit Copy impl for PublicKey (#379)

The derived impl appears to have the incorrect bounds

Author: tarcieri

elliptic-curve/src/public_key.rs

@@ -20,7 +20,7 @@ use generic_array::ArrayLength;
 ///
 /// These are a thin wrapper around [`AffinePoint`] which simplifies
 /// encoding/decoding.
-#[derive(Copy, Clone, Debug)]
+#[derive(Clone, Debug)]
 pub struct PublicKey<C>
 where
     C: Curve + ProjectiveArithmetic,
@@ -185,6 +185,15 @@ where
     }
 }
 
+impl<C> Copy for PublicKey<C>
+where
+    C: Curve + ProjectiveArithmetic,
+    FieldBytes<C>: From<Scalar<C>> + for<'r> From<&'r Scalar<C>>,
+    Scalar<C>: PrimeField<Repr = FieldBytes<C>>,
+    AffinePoint<C>: Copy + Clone + Debug,
+{
+}
+
 impl<C> Eq for PublicKey<C>
 where
     C: Curve + ProjectiveArithmetic,