CStr: derive PartialEq, Eq; add test for Ord
While working on #128046, I've spotted a peculiarity: `CStr` has `PartialEq, Eq, PartialOrd, Ord` implemented manually and not derived.
While we can't derive `PartialOrd, Ord` (due to inner `[c_char]` being `[i8]` or `[u8]` on different platforms), we *can* derive `PartialEq, Eq` (I think), allowing as to remove `#[allow(clippy::derived_hash_with_manual_eq)]` as well.
(I really hope `c_char: Eq` on all platforms)