Add _Cons and _Snoc instances for (Dual s) (Dual t) a b

[Taneb]

This allows treating a cons-list as a snoc-list in a reasonable way and vice versa

One use case for this is Dual [a] can be used as a snoc list in an efficient way, if a snoc list is more "semantically" correct.

[RyanGlScott]

Is it commonplace to interpret Dual [a] as a snoc list? I'm accustomed to thinking of Dual in the "dual monoid" sense, but these instances seem to be giving Dual an additional meaning, in the sense that it treats Cons and Snoc as dual operations. Perhaps this is a sensible thing to do, but I can't recall having seen something like this before.

[Taneb]

I don't know if it's used often, but I reached for it today, thinking of its Monoid instance

[phadej]

I think this is on edge of being too ad-hoc, and I'm not sure on which side. _Empty, _Cons, Monoid aren't "officially" related, but (I guess) are true for all instances (of Cons). If we had Singleton we could tie this all together formally, but otherwise it is a collection of names which accidentally make sense together.