Fix crashes in find_univpoly_ring and related functions

Fix crashes in find_univpoly_ring and related functions
* include/cln/modinteger.h: Remove vptr from cl_heap_modint_ring;
remove declaration of cl_class cl_class_modint_ring.
* include/cln/univpoly.h: Remove vptr from cl_heap_univpoly_ring;
remove declaration of cl_class_univpoly_ring.
* include/cln/object.h: cl_class_flags_modint_ring: New #define...
* src/modinteger/cl_MI.cc: ...used in cl_class_modint_ring.
* src/modinteger/cl_MI_fix16.h: No vptr, but static dtor and type flag.
* src/modinteger/cl_MI_fix29.h: Likewise.
* src/modinteger/cl_MI_fix32.h: Likewise.
* src/modinteger/cl_MI_int32.h: Likewise.
* src/modinteger/cl_MI_montgom.h: Likewise.
* src/modinteger/cl_MI_pow2: Likewise.
* src/modinteger/cl_MI_pow2m1.h: Likewise.
* src/modinteger/cl_MI_pow2p1.h: Likewise.
* src/modinteger/cl_MI_std.h: Likewise.
* src/polynomial/elem/cl_UP.cc (cl_make_univpoly_ring): Compare with
cl_class_flags_modint_ring, not with cl_class_modint_ring.
* src/polynomial/elem/cl_UP_GF2.h (cl_class_num_univpoly_ring): New.
* src/polynomial/elem/cl_UP_MI.h (cl_class_modint_univpoly_ring): New.
* src/polynomial/elem/cl_UP_gen.h (cl_class_gen_univpoly_ring): New.
* src/polynomial/elem/cl_UP_number.h (cl_class_num_univpoly_ring): New.
Reported by Ralf Goertz <R_Goertz@web.de>.