@:forward sugar not working for abstracts

[kevinresol]

Because in tink_macro getFields for abstract is not implemented

[kevinresol]

Snippet:

@:tink
class Main {
    
    // uncomment either one of the following
    // @:forward var a:Abs; // fail
    // @:forward var c:Cls; // ok
    
    public static function main() {
	   var m = new Main();
       trace(m.stringify());
   }
   
   function new() {}
}

abstract Abs(Int) {
    public function stringify() return 'foo';
}

class Cls {
    public function stringify() return 'foo';
}