* Remove the `empty` constructors for DFAs and replace them with
  `never_match` and `always_match` constructors.
* Consider refactoring the NFA representation such that it can be instantly
  loaded from a `&[u8]`, just like a sparse DFA. Main downside is that this
  could negatively impact using the NFA with deserialization costs. Before
  doing this, we should write PikeVM and backtracking implementations so that
  they can be benchmarked.
* Add captures and anchors to NFA.
* Once we're happy, re-organize the public API such that NFAs are exported
  and usable on their own.