Use TreeLike to implement for_each_key

Remove recursive calls and use `TreeLike`'s post order iterator to
implement `for_each_key` for the `concrete::Policy`.

No additional unit tests required, this code path is already tested.

Author: tcharding

src/policy/concrete.rs

@@ -544,30 +544,14 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
 
 impl<Pk: MiniscriptKey> ForEachKey<Pk> for Policy<Pk> {
     fn for_each_key<'a, F: FnMut(&'a Pk) -> bool>(&'a self, mut pred: F) -> bool {
-        self.real_for_each_key(&mut pred)
+        self.pre_order_iter().all(|policy| match policy {
+            Policy::Key(ref pk) => pred(pk),
+            _ => true,
+        })
     }
 }
 
 impl<Pk: MiniscriptKey> Policy<Pk> {
-    fn real_for_each_key<'a, F: FnMut(&'a Pk) -> bool>(&'a self, pred: &mut F) -> bool {
-        match *self {
-            Policy::Unsatisfiable | Policy::Trivial => true,
-            Policy::Key(ref pk) => pred(pk),
-            Policy::Sha256(..)
-            | Policy::Hash256(..)
-            | Policy::Ripemd160(..)
-            | Policy::Hash160(..)
-            | Policy::After(..)
-            | Policy::Older(..) => true,
-            Policy::Threshold(_, ref subs) | Policy::And(ref subs) => {
-                subs.iter().all(|sub| sub.real_for_each_key(&mut *pred))
-            }
-            Policy::Or(ref subs) => subs
-                .iter()
-                .all(|(_, sub)| sub.real_for_each_key(&mut *pred)),
-        }
-    }
-
     /// Converts a policy using one kind of public key to another type of public key.
     ///
     /// For example usage please see [`crate::policy::semantic::Policy::translate_pk`].