Allow type transformations with mapper for Map

Matching the return type of the mapper function with the return types of the given map functions F and G, allows the mapper function to be used to transform key and value types.

Previous definition only works for map functions that return the same type as they are given.

src/org/rascalmpl/library/Map.rsc

@@ -157,7 +157,7 @@ int incr(int x) { return x + 1; }
 mapper(("apple": 1, "pear": 2, "orange": 3), prefix, incr);
 ```
 }
-public map[&K, &V] mapper(map[&K, &V] M, &L (&K) F, &W (&V) G)
+public map[&L, &W] mapper(map[&K, &V] M, &L (&K) F, &W (&V) G)
  = (F(key) : G(M[key]) | &K key <- M);