Add checks for Foldable

[garyb]

I think Foldable is technically lawless, but these look like good properties to have even if they're not officially required. Thanks!

[jacereda]

I thought it had laws because http://hackage.haskell.org/package/base-4.9.0.0/docs/Data-Foldable.html states that

Foldable instances are expected to satisfy the following laws:

foldr f z t = appEndo (foldMap (Endo . f) t ) z foldl f z t = appEndo (getDual (foldMap (Dual . Endo . flip f) t)) z fold = foldMap id

and

If the type is also a Functor instance, it should satisfy

foldMap f = fold . fmap f

[jacereda]

Oh, right. I'll remove it.