feat: use `Q: ?Sized + Ord + equivalent::Comparable<K>` instead of `K: Borrow<Q>, Q: ?Sized + Ord` as lookup APIs trait bounds

[al8n]

Hi, may I ask about using equivalent::Comparable as lookup API trait bounds? It is something like what indexmap::IndexMap::get do.

K: Borrow<Q> is quite limit for some scenarios, an example can be found here crossbeam-rs/crossbeam#1133.