Tidy up sais.rs

[ajalab]

It's not obvious for readers that the module and function name sais computes a suffix array. Maybe it should be something like suffix_array instead.

Also, some functions in sais module like count_chars and get_bucket_start_pos are not specific to SA-IS algorithm nor suffix array. FMIndex::create uses these functions to build cs, but this misleads readers that the cs structure construction is related to SA-IS. Maybe these functions should reside in characters.rs or another module.

[faassen]

Both sounds like good steps forward. I think cleaning up code is often a good way to start on something tricky, which SA-IS definitely is.