Improve accuracy of constraint system violation diagnostics.

src/zcash/IncrementalMerkleTree.hpp

@@ -79,6 +79,7 @@ public:
     Hash root() const {
         return root(Depth, std::deque<Hash>());
     }
+    Hash last() const;
 
     IncrementalWitness<Depth, Hash> witness() const {
         return IncrementalWitness<Depth, Hash>(*this);
@@ -138,6 +139,12 @@ public:
         return tree.path(partial_path());
     }
 
+    // Return the element being witnessed (should be a note
+    // commitment!)
+    Hash element() const {
+        return tree.last();
+    }
+
     Hash root() const {
         return tree.root(Depth, partial_path());
     }