sort_unstable_by has no fallback to equal

[AndriBaal]

I use delaunator in a project of mine and sometimes on specific conditions the delaunator crashes because of the partial_cmp of 2 floats without a fallback. Specifically, this needs to be changed:

fn sortf(f: &mut [(usize, f64)]) {
    f.sort_unstable_by(|&(_, da), &(_, db)| da.partial_cmp(&db).unwrap());
}

to:

fn sortf(f: &mut [(usize, f64)]) {
    f.sort_unstable_by(|&(_, da), &(_, db)| da.partial_cmp(&db).unwrap_or(core::cmp::Ordering::Equal));
}

[mourner]

@AndriBaal nice catch! Would you mind making a PR with a fix + test?

[mourner]

Looks like this would fix #9 too.

[AndriBaal]

I have created a pull request, let me know if anything is missing. #28

[mourner]

Fixed by #28