Merge #617: Improve TapTree API

0630491 Make TapTree::height public (Tobin C. Harding)
cdb57cb Make TapTree::combine public (Tobin C. Harding)

Pull request description:

  Improve the `TapTree` API by making the `height` and `combine` methods public.

  Note the serendipitous finding that upgrading miniscript to use bitcoin v0.31.0-rc1 requires no changes. I found these fixes doing the `bdk` upgrade.

ACKs for top commit:
  apoelstra:
    ACK 0630491

Tree-SHA512: bafb91cc34e353bab7b952f8cfa8674f4918422879b0514d1bc0f7a2e00af3d5103f133eb65488ad3ff96ea0801292b863c74d1e2523fe1d67dea8ce8fb662af

Author: apoelstra

src/descriptor/tr.rs

@@ -118,13 +118,13 @@ impl<Pk: MiniscriptKey> hash::Hash for Tr<Pk> {
 
 impl<Pk: MiniscriptKey> TapTree<Pk> {
     /// Creates a `TapTree` by combining `left` and `right` tree nodes.
-    pub(crate) fn combine(left: TapTree<Pk>, right: TapTree<Pk>) -> Self {
+    pub fn combine(left: TapTree<Pk>, right: TapTree<Pk>) -> Self {
         let height = 1 + cmp::max(left.height(), right.height());
         TapTree::Tree { left: Arc::new(left), right: Arc::new(right), height }
     }
 
     /// 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,