To make RClass size smaller, move flags of prime classext readable/writable to:
readable - use ns_classext_tbl is NULL or not (if NULL, it's readable)
writable - use FL_USER2 of RBasic flags
`rb_define_private_method` performs strict type checking on the function
pointer. As a result, we cannot pass the function a generic signature.
```
/github/workspace/src/namespace.c:1097:72: note: expected 'VALUE (*)(void)' {aka 'long unsigned int (*)(void)'} but argument is of type 'VALUE (*)(int, VALUE *, VALUE)' {aka 'long unsigned int (*)(int, long unsigned int *, long unsigned int)'}
1097 | namespace_define_loader_method(VALUE module, const char *name, VALUE (*func)(ANYARGS), int argc)
| ~~~~~~~~^~~~~~~~~~~~~~
```
This commit defines the method directly to avoid the mismatch error.