bitfield support

```C++
union u {
	long u_l;
	char u_c;
	long u_b: 2;
	S1 s1;
};
```

will be translated to:

```C
union HidvaUnion3 {
  long int u_l;
  char u_c;
  __u8 __bitfield0[1];   // #1
  struct HidvaStruct1 s1;
  __u8 __HIDVA_dont_use2[16];
} __attribute__((__packed__));
```