**TODO: document this **

The implementation must respect the FGW_DYN type flag; in a multicall,
the caller will make non-dynamic copies that shall never be free'd, even
if the custom type sotres the payload as a separate allocation. In that
case the non-dynamic copies will exist only as long as the dynamic parent
copy exists (the caller gunaratees this) and they will all point to the
same allocation. This also means a conversion to a different type
shall not change the memory behind the allocation.