`generate_edits` API can be reworked to avoid `Result`

The `generate_edits` function currently only returns an `Err` if the matrix passed in is incorrect in some way. In my opinion, it makes much more sense to remove that parameter, call `distance` internally, and return only the `Vec>`.

The downside of doing it this way is that the user can no longer select which version of the levenshtein functions they use. However, I don't see that as a major downside compared to simplifying the API.

I'm willing to write a PR for this if you agree with the idea.