Implement the BitSet API

[arucil]

The following set operations are what I can think of currently.

iter_ones
set difference
collect::<BitSet>() from an iterator of usize's

[myrrlyn]

The difference between two slices can be computed with the ^ or ^= operators. At present, these are shims over

use bitvec::prelude::*;

let a = bits![mut 0, 0, 1, 1];
let b = bits![0, 1, 0, 1];

for (mut a, b) in a.iter_mut().zip(b.iter().copied()) {
  *a ^= b;
}

as I have not yet implemented specialization for batch comparison.

The FromIterator<T> implementation, as well as the .iter_ones() and .iter_zeros() methods, will be available in the next release.

If you have any other requests, please feel free to reöpen this issue.