Key constraint might not be ideal

I have [a wrapper of immutable](https://github.com/anacrolix/stm/tree/master/stmutil) that adds sets and some other generic interface stuff. I'm [having difficulty](https://github.com/anacrolix/stm/issues/5#issuecomment-1291807139) mapping it onto v0.4.0 of immutable. I read [the PR ](https://github.com/benbjohnson/immutable/pull/23) for the generics, and I think that constraints.Ordered isn't quite right for the key values. I haven't looked into it more, but I suspect it's too strict and that `comparable` should be possible instead, particularly because the user already has to provide comparer/hasher types in the constructors.

@laher