Allow SharedSecret to be created from byte array

This was accidentally removed in 8b2edad. See also the discussion
on rust-bitcoin#402

src/ecdh.rs

@@ -47,6 +47,14 @@ const SHARED_SECRET_SIZE: usize = constants::SECRET_KEY_SIZE;
 pub struct SharedSecret([u8; SHARED_SECRET_SIZE]);
 impl_display_secret!(SharedSecret);
 
+impl From<[u8; SHARED_SECRET_SIZE]> for SharedSecret {
+    fn from(arr: [u8; SHARED_SECRET_SIZE]) -> Self {
+        let mut data = [0u8; SHARED_SECRET_SIZE];
+        data[..SHARED_SECRET_SIZE].copy_from_slice(&arr);
+        Self(data)
+    }
+}
+
 impl SharedSecret {
     /// Creates a new shared secret from a pubkey and secret key.
     #[inline]