this reuses the same notation for the original_type tag and the type length. This will be ambiguous if we use both.
what about the following syntax instead?
```
 optional group b (MAP) {
   repeated group map (MAP_KEY_VALUE) {
      required binary key;
      optional group value {
        optional fixed_len_byte_array(5) c (UTF8);
        optional fixed_len_byte_array(7) d (UTF8);
      }
    }
 }
```
also if those are UTF8 encoded, they should use the UTF8 original type tag.