tr: Harmonize Display and FromStr

Display is the basis for ToString.
FromStr expects sim{...} around Simplicity tap leaves.
This commit serializes Simplicity leaves in the same way.
The commit also tests this behavior.

Author: uncomputable

src/descriptor/tr.rs

@@ -234,7 +234,7 @@ impl<Pk: MiniscriptKey, Ext: Extension> fmt::Display for TapTree<Pk, Ext> {
         match self {
             TapTree::Tree(ref left, ref right) => write!(f, "{{{},{}}}", *left, *right),
             TapTree::Leaf(ref script) => write!(f, "{}", *script),
-            TapTree::SimplicityLeaf(ref policy) => write!(f, "{}", policy),
+            TapTree::SimplicityLeaf(ref policy) => write!(f, "sim{{{}}}", policy),
         }
     }
 }
@@ -949,15 +949,17 @@ mod tests {
     fn tr_from_str() {
         // Key spend only
 
-        let desc = "eltr(internal)";
-        let desc = Tr::<String, NoExt>::from_str(desc).unwrap();
+        let desc_str = "eltr(internal)#0aen4jhp";
+        let desc = Tr::<String, NoExt>::from_str(desc_str).unwrap();
+        assert_eq!(desc_str, &desc.to_string());
         assert_eq!("internal", desc.internal_key);
         assert!(desc.iter_scripts().next().is_none());
 
         // Miniscript key spend
 
-        let desc = "eltr(internal,pk(a))";
-        let desc = Tr::<String, NoExt>::from_str(desc).unwrap();
+        let desc_str = "eltr(internal,pk(a))#vadmk9gd";
+        let desc = Tr::<String, NoExt>::from_str(desc_str).unwrap();
+        assert_eq!(desc_str, &desc.to_string());
         assert_eq!("internal", desc.internal_key);
 
         let scripts: Vec<_> = desc.iter_scripts().collect();
@@ -967,8 +969,9 @@ mod tests {
 
         // Simplicity key spend
 
-        let desc = "eltr(internal,sim{pk(a)})";
-        let desc = Tr::<String, NoExt>::from_str(desc).unwrap();
+        let desc_str = "eltr(internal,sim{pk(a)})#duhmnzmm";
+        let desc = Tr::<String, NoExt>::from_str(desc_str).unwrap();
+        assert_eq!(desc_str, &desc.to_string());
         assert_eq!("internal", desc.internal_key);
 
         let policy = desc.get_simplicity().expect("single simplicity leaf");