Support unnamed flags

[KodrAus]

This PR introduces the concept of unnamed flags. You can add an unnamed flag to your flags types like:

bitflags! {
    pub struct Flags: u8 {
        // regular, named flag
        const A = 1;

        // unnamed flag
        const _ = !0;
    }
}

Adding an unnamed flag means its bits will be part of the mask in from_bits_truncate, but it won't generate a constant, and it won't be given a name when formatting or parsing.