Fix #9: Collect sub-rules in list of dictionaries

This commit adds support for collect rules:

* collect x:
  - u in ['a', 'b'];
  - v in [1, 2];

In this example, x is bound to the list [{ u: 'a', v: 1}, { u: 'a', v: 2}, { u: 'b', v: 1}, { u: 'b', v: 2}].