Remove bitmap! in favor of #[bitmap]

[winstonallo]

I wanted to have both, but that would have required one of them to be named gen_bitmap! or #[bitmap_attr], which I did not like, so I removed bitmap! in favor of #[bitmap], which feels closer to actual Rust syntax and more intuitive.

The macro still generates the same API and should now be used like this:

#[bitmap]
struct Bits {
    flag: u1,
    counter: u7, 
}

Closes #8