Mixed compositions

[S11001001]

In #348, I found that, say, the product of Traverse and Traverse1 turns out to be Traverse1. So these are "mixed" composes and products for Contravariant, Functor, Foldable[1], and Traverse[1]. They have funny names I chose mainly to avoid the tarpit of overloading.

[larsrh]

Could you add tests for these?

[S11001001]

Sure; it will require formulating some laws for Foldable, Foldable1, and Traverse1 first.

[larsrh]

I realized that requesting tests was a bit silly. I hope we can get some laws for at least Traverse1 in the near future. I suspect that Foldable has no laws beyond parametricity, though.

[pthariensflame]

@larsrh In Scala, parametricity is a non-trivial (and testable) law-set, though.