refactor(rs): fix linter warnings in distances.rs

[Neclow]

• Refactored the loop that inserts leaves into their own descendant sets in get_bipartitions to use .iter_mut().enumerate().take(n_leaves) for better readability and idiomatic Rust.
• Updated assertions in tests to use the more idiomatic contains method with range patterns instead of chained comparisons for checking value ranges. [1] [2]