tr: move Liftable impl for TapTree

Moving this separately from the rest of the TapTree stuff because I
changed the Policy import to Semantic and the Error import to just
crate::Error (since this is the only place that type is used in the
taptree module, and I don't really like this type).

So this is a move-only diff but it might not look like it, so I figured
I should at least make the diff small.

Author: apoelstra

src/descriptor/tr/mod.rs

@@ -480,22 +480,6 @@ impl<Pk: MiniscriptKey> fmt::Display for Tr<Pk> {
     }
 }
 
-impl<Pk: MiniscriptKey> Liftable<Pk> for TapTree<Pk> {
-    fn lift(&self) -> Result<Policy<Pk>, Error> {
-        fn lift_helper<Pk: MiniscriptKey>(s: &TapTree<Pk>) -> Result<Policy<Pk>, Error> {
-            match *s {
-                TapTree::Tree { ref left, ref right, height: _ } => Ok(Policy::Thresh(
-                    Threshold::or(Arc::new(lift_helper(left)?), Arc::new(lift_helper(right)?)),
-                )),
-                TapTree::Leaf(ref leaf) => leaf.lift(),
-            }
-        }
-
-        let pol = lift_helper(self)?;
-        Ok(pol.normalized())
-    }
-}
-
 impl<Pk: MiniscriptKey> Liftable<Pk> for Tr<Pk> {
     fn lift(&self) -> Result<Policy<Pk>, Error> {
         match &self.tree {

src/descriptor/tr/taptree.rs

@@ -5,9 +5,10 @@ use core::{cmp, fmt};
 use bitcoin::taproot::{LeafVersion, TapLeafHash};
 
 use crate::miniscript::context::Tap;
+use crate::policy::{Liftable, Semantic};
 use crate::prelude::Vec;
 use crate::sync::Arc;
-use crate::{Miniscript, MiniscriptKey, ToPublicKey, TranslateErr, Translator};
+use crate::{Miniscript, MiniscriptKey, Threshold, ToPublicKey, TranslateErr, Translator};
 
 /// A Taproot Tree representation.
 // Hidden leaves are not yet supported in descriptor spec. Conceptually, it should
@@ -69,6 +70,22 @@ impl<Pk: MiniscriptKey> TapTree<Pk> {
     }
 }
 
+impl<Pk: MiniscriptKey> Liftable<Pk> for TapTree<Pk> {
+    fn lift(&self) -> Result<Semantic<Pk>, crate::Error> {
+        fn lift_helper<Pk: MiniscriptKey>(s: &TapTree<Pk>) -> Result<Semantic<Pk>, crate::Error> {
+            match *s {
+                TapTree::Tree { ref left, ref right, height: _ } => Ok(Semantic::Thresh(
+                    Threshold::or(Arc::new(lift_helper(left)?), Arc::new(lift_helper(right)?)),
+                )),
+                TapTree::Leaf(ref leaf) => leaf.lift(),
+            }
+        }
+
+        let pol = lift_helper(self)?;
+        Ok(pol.normalized())
+    }
+}
+
 impl<Pk: MiniscriptKey> fmt::Display for TapTree<Pk> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {