Use TreeLike to implement keys

Remove recursive calls and use `TreeLike`'s post order iterator to get
all the keys from a `concrete::Policy`.

Author: tcharding

src/policy/concrete.rs

@@ -668,19 +668,13 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
 
     /// Gets all keys in the policy.
     pub fn keys(&self) -> Vec<&Pk> {
-        match *self {
-            Policy::Key(ref pk) => vec![pk],
-            Policy::Threshold(_k, ref subs) => {
-                subs.iter().flat_map(|sub| sub.keys()).collect::<Vec<_>>()
+        let mut keys = vec![];
+        for data in self.post_order_iter() {
+            if let Policy::Key(ref pk) = data.node {
+                keys.push(pk);
             }
-            Policy::And(ref subs) => subs.iter().flat_map(|sub| sub.keys()).collect::<Vec<_>>(),
-            Policy::Or(ref subs) => subs
-                .iter()
-                .flat_map(|(ref _k, ref sub)| sub.keys())
-                .collect::<Vec<_>>(),
-            // map all hashes and time
-            _ => vec![],
         }
+        keys
     }
 
     /// Gets the number of [TapLeaf](`TapTree::Leaf`)s considering exhaustive root-level [`Policy::Or`]