Bitfield woes

[SealabJaster]

I swear I'm not being stupid right: https://run.dlang.io/is/Wfd214

I should be seeing:

```
7 // 111
7 // 111
3 // 11
[no error message]
```

instead of:

```
3 // 11
3 // 11
1 // 1
core.exception.AssertError at onlineapp.d-mixin-8(9): Value is 
greater than the maximum value of bitfield 'a'
```

Or am I managing to miss something completely obvious?