Moves `cusp_for_hue`, `max_chroma_at_lightness`, `toe`, and `toe_inv` from `Oklab` inherent methods to free functions in the `oklab` module. These utilities are shared across Okhsl and Okhsv and are not specific to the Oklab struct itself. Call sites in `okhsl.rs` and `okhsv.rs` now import and call the free functions directly instead of going through `Oklab::`.