Rename `mnat`/`mnat_auth` into `mono_nat`.
- This avoids confusion between `mnat` and `max_nat`. The `m` stands for `mono`. - With `_mono` added, the `_auth` suffix in the algebra name no longer makes sense, so I removed it. - This makes the names between the logic and the algebra-level library consistent. - I also renamed `_frag` into `_lb` in the algebra-level library so as to make it consistent with the logic-level library. Furthermore make the order of lemmas consistent and make the versions for the fractions consistent.