taproot: add TapNodeHash getter method on TapTree and NodeInfo

Fixes a gap in the API of the taproot module. Callers can now use
TapTree::root_hash or NodeInfo::node_hash to extract the taproot
tree merkle root hash for fast validation without any ECC overhead.

bitcoin/src/taproot/mod.rs

@@ -693,6 +693,9 @@ impl TapTree {
     /// Returns [`TapTreeIter<'_>`] iterator for a taproot script tree, operating in DFS order over
     /// tree [`ScriptLeaf`]s.
     pub fn script_leaves(&self) -> ScriptLeaves { ScriptLeaves { leaf_iter: self.0.leaf_nodes() } }
+
+    /// Returns the root [`TapNodeHash`] of this tree.
+    pub fn root_hash(&self) -> TapNodeHash { self.0.hash }
 }
 
 impl TryFrom<TaprootBuilder> for TapTree {
@@ -842,6 +845,9 @@ impl NodeInfo {
 
     /// Creates an iterator over all leaves (including hidden leaves) in the tree.
     pub fn leaf_nodes(&self) -> LeafNodes { LeafNodes { leaf_iter: self.leaves.iter() } }
+
+    /// Returns the root [`TapNodeHash`] of this node info.
+    pub fn node_hash(&self) -> TapNodeHash { self.hash }
 }
 
 impl TryFrom<TaprootBuilder> for NodeInfo {