Add convenience function registered::SecretKey::derive_child.

Signed-off-by: Daira-Emma Hopwood <daira@jacaranda.org>

src/registered.rs

@@ -119,6 +119,14 @@ impl SecretKey {
         })
     }
 
+    /// Derives a child key from a parent key at a given index and empty tag.
+    ///
+    /// This is a convenience function equivalent to
+    /// `self.derive_child_with_tag(index, &[])`.
+    pub fn derive_child(&self, index: ChildIndex) -> Self {
+        self.derive_child_with_tag(index, &[])
+    }
+
     /// Derives a child key from a parent key at a given index and (possibly empty) tag.
     ///
     /// Defined in [ZIP 32: Registered child key derivation][regckd].
@@ -167,7 +175,7 @@ pub fn cryptovalue_from_subpath(
     assert!(!subpath.is_empty());
 
     let mut xsk = SecretKey::master(context_string, seed)
-        .derive_child_with_tag(ChildIndex::hardened(u32::from(zip_number)), &[]);
+        .derive_child(ChildIndex::hardened(u32::from(zip_number)));
 
     for (i, tag) in subpath.iter().take(subpath.len() - 1) {
         xsk = xsk.derive_child_with_tag(*i, tag);