Tidy up sais module

[ajalab]

Related: #48

This PR

• moves sais module under suffix_array module
• renames sais() function to build_suffix_array for better code readability.

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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.

This part mentioned in #48 has not been addressed in this PR yet. If we move count_chars and get_bucket_start_pos to characters module, a circular dependency occurs:

• characters module depends on converter module, as count_chars requires Converter.
• converter module depends on character module, as Converter requires Character.

While this might be handled by Rust language well, it'd be better to avoid this kind of complexity if possible. One option is to reorganize character module like below.

Module	Consists of	Depends on
character	Character	-
character::count	count_chars, get_bucket_start_pos	character::Character
character::converter	Converter	character::Character

Anyway, let me postpone relocating count_charts and get_bucket_start_pos for now.

[faassen]

Just as a small comment: I haven't had problems with circular dependencies in Rust. But of course from a design perspective it makes sense to have things be sense contained. I like the organization in your table, as it's logical converter is related to character. I'm not a big fan of putting code other than use and pub statements in a mod.rs, so I'd be inclined to have a character::core module that contains Character. And then the mod.rs just exports everything relevant to the rest of the crate.

[faassen]

this is definitely a lot easier to read! It took me a while to figure out that this was a suffix array and now it's immediately obvious.