code: func decodeStructHashGetter(mrb *Mrb, h *Hash) decodeStructGetter {
  	return func(key string) (*MrbValue, error) {
  		rbKey := mrb.StringValue(key)
  		return h.Get(rbKey)
  	}
  }
docs: // decodeStructHashGetter is a decodeStructGetter that reads values from
  // a hash.