Relax some trait bounds on BinaryHeap<T, C>

This is inspired by rust-lang/rust#58421: "Relax some `Ord` bounds on
`BinaryHeap<T>`", which split out the methods of `BinaryHeap<T>` which
do not require the bound `T: Ord` to a separate impl block.

Note that in order to do something similar here, we also have to remove
the trait bound `C: Compare<T>` from the definition of the struct
`BinaryHeap<T, C>`; the upstream definition of `BinaryHeap<T>` did not
have the analogous bound `T: Ord` on the struct definition in the first
place.