Add support for nested loops

[wcmjunior]

Currently, nested loops are not supported, but examples like the following should be made possible:

    array1.par().foreach(new UserFunction<Type1>() {
        public void function(Type1 tp1) {
            array2.par().foreach(new UserFunction<Type2>() {
                public void function(Type2 tp2) {
                    type2.someProperty = type1.someProperty;
                    ...
                }
            });
        }
    });