Change syntax for impl

Move the name of the bundle to the front, allow type parameters (not
handled yet), and add a 'for' keyword:

    impl utils for int {
        fn str() -> str { int::str(self) }
        fn times(f: block()) { ... }
    }

src/comp/syntax/ast.rs

@@ -505,7 +505,7 @@ tag item_ {
              node_id /* dtor id */,
              [ty_param],
              node_id /* ctor id */);
-    item_impl(@path /* iface */, @ty /* self */, [@method]);
+    item_impl([ty_param], @ty /* self */, [@method]);
 }
 
 type native_item =