When a bitflags type contains composite flags like:

```rs
bitflags! {
    struct Flags: u8 {
            const A = 0b00000001;
            const BC = 0b00000110;
    }
}
```

from_bits and from_bits_truncate would not work as expected allowing flags that are not declared.

Fixes bitflags#275

extra brace commited accidentally due to partial staging

serde:: doesn't work on tests on CI

…from_bits

was still wrong for some edge cases

changed on last commit