semantic: Remove recursion in for_each_key

Done as part of the effort to remove all the recursion crate wide.

Use the `TreeLike` trait to iterate over policy nodes and remove the
recursive call in `semantic::Policy::for_each_key`.

Author: tcharding

src/policy/semantic.rs

@@ -66,27 +66,14 @@ where
 
 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) => {
-                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.
     ///
     /// # Examples