Add count_ones_to_offset function

[fuine]

I have implemented function: fn count_ones_to_offset(&self, offset: usize) -> usize. I would like to add it to this crate, but when I asked @bluss about it, he suggested that maybe this function should take Range as argument. Example usage below:

    let mut fb = FixedBitSet::with_capacity(15);
    fb.set(11, true);
    fb.set(7, true);
    assert_eq!(fb.count_ones_to_offset(6), 0);
    assert_eq!(fb.count_ones_to_offset(7), 1);
    assert_eq!(fb.count_ones_to_offset(11), 2);

[bluss]

fb.count_ones(..), fb.count_ones(..6), fb.count_ones(3..11) seems like the most powerful and simple to use API. However, that would require some kind of range trait. You can copy the code from crate odds.

[bluss]

Or just implement it for Range<usize> for now.

[fuine]

I thought about using this crate like so:
fn count_ones<T: Num>(&self, r: Range<T>) -> usize. What do you think?

[bluss]

Absolutely not. Nothing personal, but indices are only usize in Rust, we don't need numerical traits.

[fuine]

Sure. I think i will use the trait from odds, do you want me to copy the implementation over or include odds as dependency? Also i think that in the long term we could migrate to rust-lang/rust#30877 if it ever stabilizes

[bluss]

copy