Restrict (co)recursive instances

[sellout]

This uses a Strict type family to tag types and type constructors with their strictness, allowing instances to be constrained more precisely.

This mitigates the issue that using a non-strict functor with, say, Mu will result in potentially infinite structures, since any functor used with Mu must be Strict in order to be folded.

loose ends

Before this PR is complete, the yaya-unsafe:Yaya.Unsafe.Fold.Instances module will go away in favor of instances defined over a newtype Unsafe t that will live directly in yaya-unsafe:Yaya.Unsafe.Fold. E.g.,

instance (Projectable (->) t f) => Projectable (->) (Unsafe t) f where
  project (Unsafe t) = project t

instance Recursive (Unsafe (Nu f)) f where
  cata φ (Unsafe nu) = unsafeCata φ nu

[sellout]

This is the main new thing added, and this needs instances for many cases.

On the plus side, if an instance is missing, the type that’s missing an instance won’t work with anything, so they can be added as needed.

[sellout]

Strict goes through a few layers, and is used here (and in Corecursive) to ensure the involved types have the required strictness.

[sellout]

This is an interesting consequence – when the functor is non-strict, Fix can be used corecursively. However, the same can’t be done with Mu, because embed (and project) on Mu requires cata, which has the IsStrict (Mu f) constraint, which would conflict with the IsNotStrict (Mu f) constraint required by ana.