Changes in the work include:
- [x] Internally use linear_space for iterating
- [x] Simplify type and value iteration in `state_iterator::build_axis_configs`
- [x] Store the iteration space in `axes_metadata`
- [x] Expose `tie` and `user` spaces to user
- [x] Add tests for `linear`, `tie`, and `user`
- [x] Add examples for `tie` and `user`