Fix: NSUInteger and unsigned int different size on 64-bit systems.

On a 64-bit system the value of `methodCount` returned will be something like 4294967296.

If you're lucky you'll get as far as an error message trying to use it. Yes, this is an actual error message!

```
'*** -[__NSPlaceholderSet initWithCapacity:]: capacity (18446744065119617024) is ridiculous'
```