replace/migrate from libsais

[whym]

This program used https://sites.google.com/site/yuta256/sais by Yuta Mori via JNI for faster computation than pure Scala. It looks like the library is not publicly available anymore.

I believe there are comparable libraries implemented in C or nowadays in Rust, if not an exact replacement.

Alternatively, I or someone else might be able to obtain and redistribute sais here. I recall that the software was free in some sense. (GPL, Apache, etc.)