Also replace derived types (map_type, collection_type, etc.).
As we'll change data_type's definition, this reduces the number of places
that need to be modified later, and is more readable.
We didn't handle properly the case when the last component of a
prefixable compound was empty. Because we do not encode component's
length, we did not distinguish a compound with last element empty from
a compound without the last element.
The fix is to always encode lengths in prefixable tuples.
