A fixed-size bitset type with a partially dense, partially sparse
representation. The bitset is broken into chunks, and chunks that are all
zeros or all ones are represented and handled very efficiently.
A bitset with a mixed representation, using DenseBitSet for small and
medium bitsets, and ChunkedBitSet for large bitsets, i.e. those with
enough bits for at least two chunks. This is a good choice for many bitsets
that can have large domain sizes (e.g. 5000+).