cmp: Use default methods in trait Ord, only require Ord::lt

It will be simpler to implement only one method for Ord, while we also
allow implementing all four Ord methods for semantics or performance
reasons.

We only supply three default methods (and not four), because don't have
any nice error reporting for the case where at least one method must be
implemented, but it's arbitrary which.
auto merge of #7765 : blake2-ppc/rust/ord-default-methods, r=huonw

Rust will allow to supply default methods for all four methods, but we
don't have any nice error reporting for the case where at least one
method must be implemented, but it's arbitrary which.

So in this case, we require `lt`, but allow implementing the others if needed.