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.
Bring hs_reset_stream(), hs_reset_and_copy_stream()'s functionality into
line with hs_close_stream() by accepting a NULL scratch if and only if
the match callback is also NULL, indicating that no matches should be
delivered.
