code: func (h *Hash) Keys() (*MrbValue, error) {
  	result := C.mrb_hash_keys(h.state, h.value)
  	return newValue(h.state, result), nil
  }
docs: // Keys returns the array of keys that the Hash has. This is returned
  // as an *MrbValue since this is a Ruby array. You can iterate over it as
  // you see fit.