Trac #30597: Define a sparse bitset structure

This structure is a copy of `bitset_t` for now and at the moment mainly
illustrates that the entire setup with a fused type in `biteset.pxi`
works.

After combinatorial polyhedron uses this type for its atom-
representation it should contain some additional information about which
limbs are relevant. So it will increase slightly in memory usage, while
still using constant memory (dependent on size). However, access to non-
zero limbs will be improved.

URL: https://trac.sagemath.org/30597
Reported by: gh-kliem
Ticket author(s): Jonathan Kliem
Reviewer(s): Travis Scrimshaw