Subkey fix and refactor (#418)

* Fix bug and refactoring

- Fix panic in some cases
- Extract the generation as a reusable function
- Improve scoring calculation
- Add tests
- Sort the results
- Fix the case when called with a desired pattern either empty or of 1 char
- Add a 'even more' paranoiac mode
- Add benchmarks

Fix #417

* Fixed compilation for nightly