Make functions cloneable for testing.

It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.

Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.

cranelift/src/libcretonne/entity_map.rs

@@ -45,7 +45,7 @@ pub trait EntityRef: Copy + Eq {
 }
 
 /// A mapping `K -> V` for densely indexed entity references.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct EntityMap<K, V>
     where K: EntityRef
 {