Add back in helper function _translate_pk

Recently while removing recursion I mistakenly removed the
`_translate_pk` function forgetting that this was a trick to prevent
compilation explosion caused by the generics.

src/policy/concrete.rs

@@ -559,6 +559,14 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
     ///
     /// For example usage please see [`crate::policy::semantic::Policy::translate_pk`].
     pub fn translate_pk<Q, E, T>(&self, t: &mut T) -> Result<Policy<Q>, E>
+    where
+        T: Translator<Pk, Q, E>,
+        Q: MiniscriptKey,
+    {
+        self._translate_pk(t)
+    }
+
+    fn _translate_pk<Q, E, T>(&self, t: &mut T) -> Result<Policy<Q>, E>
     where
         T: Translator<Pk, Q, E>,
         Q: MiniscriptKey,