Allow SharedSecret to be created from byte array

This was accidentally removed in 8b2edad. See also the discussion
on https://github.com/rust-bitcoin/rust-secp256k1/pull/402

src/ecdh.rs

@@ -71,6 +71,11 @@ impl SharedSecret {
     pub fn secret_bytes(&self) -> [u8; SHARED_SECRET_SIZE] {
         self.0
     }
+
+    /// Creates a shared secret from a byte serialization.
+    pub fn from_bytes(bytes: [u8; SHARED_SECRET_SIZE]) -> SharedSecret {
+        SharedSecret(bytes)
+    }
 }
 
 impl Borrow<[u8]> for SharedSecret {