[mc] Use reference to types intead of type IDs

We do not need to pass the type ID+object_info everywhere.
It is much simpler to pass a pointer to the type (dw_type_t):

 * remove 'info', 'other_info' parameters;

 * remove cumbersome 'switch_types' logic (code duplication);

 * remove type ID confusion in mm_diff (the type ID of each block was
   stored but not the originating ELF obejct leading to an ambiguity);

 * the code is now generalisable to more than 2 ELF objects;

 * remove type lookups (by name, by ID) which are all resolved at
   initialisation time.