drop-in type check for rb_define_method

The rb_define_method function takes a pointer to ANYARGS-ed functions,
which in fact varies 18 different prototypes.  We still need to
preserve ANYARGS for storages but why not check the consistencies if
possible.

Q&As:

Q: Where did the magic number "18" came from in the description above?

A: Count the case branch of vm_method.c:call_cfunc_invoker_func().
   Note also that the 18 branches has lasted for at least 25 years.
   See also 200e0ee2fd3c1c006c528874a88f684447215524.

Q: What is this __weakref__ thing?

A: That is a kind of function overloading mechanism that GCC provides.
   In this case for instance rb_define_method0 is an alias of
   rb_define_method, with a strong type.

Q: What is this __transparent_union__ thing?

A: That is another kind of function overloading mechanism that GCC
   provides.  In this case the attributed function pointer is either
   VALUE(*)(int,VALUE*,VALUE) or VALUE(*)(int,const VALUE*,VALUE).

   This is better than void* or ANYARGS because we can reject all
   other possibilities than the two.

Q: What does this rb_define_method macro mean?

A: It selects appropriate alias of the rb_define_method function,
   depending on the arity.

Q: Why the prototype change of rb_f_notimplement?

A: Function pointer to rb_f_notimplement is special cased in
   vm_method.c:rb_add_method_cfunc().  That should be handled by the
   __builtin_choose_expr chain inside of rb_define_method macro
   expansion.  In order to do so, comparison like (func ==
   rb_f_notimplement) is inappropriate for __builtin_choose_expr's
   expression (which must be a compile-time integer constant but the
   address of rb_f_notimplement is not fixed until the linker).  So
   instead we are using __builtin_types_compatible_p, and in doing so
   we need to distinguish rb_f_notimplement from others, by type.

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