Make TapTree::height public

There is no reason to keep this function private, the `height` field can
be gotten at by pattern matching so this function is just a convenience,
might as well let users have at it.

Author: heap-coder

src/descriptor/tr.rs

@@ -124,7 +124,7 @@ impl<Pk: MiniscriptKey> TapTree<Pk> {
     }
 
     /// Returns the height of this tree.
-    fn height(&self) -> usize {
+    pub fn height(&self) -> usize {
         match *self {
             TapTree::Tree { left: _, right: _, height } => height,
             TapTree::Leaf(..) => 0,