Prohibit Class#allocate in a different way

The method introduced by #5979 causes a fault by swapping classes.

```console
% bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
zsh: segmentation fault (core dumped)  bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
```

After applying this patch, a `TypeError` exception will be raised.

```console
% bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
trace (most recent call last):
        [1] -e:1
-e:1:in method: allocation failure of Proc (TypeError)
```

However, if the `mrb_vtype` is the same object, the same care must still be taken as before.

```console
% bin/mruby -e 'Method = Binding; p method(:puts).eval("12345")'
trace (most recent call last):
        [1] -e:1
-e:1:in eval: wrong argument type nil (expected Proc) (TypeError)
```

mrbgems/mruby-data/src/data.c

@@ -492,7 +492,8 @@ mrb_mruby_data_gem_init(mrb_state* mrb)
 {
   struct RClass *d;
   d = mrb_define_class(mrb, "Data",  mrb->object_class);
-  MRB_SET_INSTANCE_TT(d, MRB_TT_UNDEF);
+  MRB_SET_INSTANCE_TT(d, MRB_TT_STRUCT);
+  MRB_UNDEF_ALLOCATOR(d);
 
   mrb_undef_class_method(mrb, d, "new");
   mrb_define_class_method(mrb, d, "define",          mrb_data_s_def,      MRB_ARGS_ANY());