elliptic-curve: add From<SecretKey<C>> for PublicKey<C> impls (#2247)

Surprised these didn't exist already

Author: tarcieri

elliptic-curve/src/public_key.rs

@@ -2,7 +2,7 @@
 
 use crate::{
     AffinePoint, CurveArithmetic, CurveGroup, Error, NonZeroScalar, ProjectivePoint, Result,
-    point::NonIdentity,
+    SecretKey, point::NonIdentity,
 };
 use core::fmt::Debug;
 use group::Group;
@@ -293,6 +293,24 @@ where
     }
 }
 
+impl<C> From<SecretKey<C>> for PublicKey<C>
+where
+    C: CurveArithmetic,
+{
+    fn from(secret_key: SecretKey<C>) -> Self {
+        secret_key.public_key()
+    }
+}
+
+impl<C> From<&SecretKey<C>> for PublicKey<C>
+where
+    C: CurveArithmetic,
+{
+    fn from(secret_key: &SecretKey<C>) -> PublicKey<C> {
+        secret_key.public_key()
+    }
+}
+
 #[cfg(feature = "sec1")]
 impl<C> PartialOrd for PublicKey<C>
 where