Since these structures are in scratch, they do not have to be as small as possible and we can use fatbit instead of multibit to improve performance.
Remove the RoseEngine and stream state pointers frose RoseContext, as they are also present in core_info. Unify stream state handing in Rose to always use a char * (we were often a u8 * for no particularly good reason) and tidy up.
Analogous to q_cur_type(), asserts that queue indices are within a valid range.
