add InvariantK instance for Mid

[GreyPlane]

I found it quite useful for example because I'm using Future and wanna implement a Cache middleware so I have to make my effect lazily by Lazy[Future], so my type will be Service[Mid[Lambda[a => Lazy[Future[a]], *]], buy my Metrics middleware remains Future, so instead of change Metrics' effect type to Lazy[Future] and manually wrap and unwrap Lazy in every call, I think it is a lot easier to just map Mid[Future, *] to Mid[Lambda[a => Lazy[Future[a]], *] which can be done by InvariantK.imapK.