Explicitly prohibit bitfields or generate glue code for them

One aspect of C I completely forgot about is bitfields, e.g.

```c
struct Flags {
    unsigned int a : 1;
    unsigned int b : 5;
    unsigned int c : 13;
};
```

The layout is not guaranteed, accessing it is weird, size is not defined..

I think we should outright reject them, because layout is compiler specific we cannot expose this in bindings.