Make StackSafeMonad extend Defer

[johnynek]

Make StackSafeMonad extend Defer.

See the discussion in #3790 for more context.

The argument for why this is safe is given in the comments on StackSafeMonad. If they don't seem to make sense, please add a comment there.

cc @djspiewak @non

I think we can follow up with using an idea similar to shift from Kleisli to make an Applicative map2 abstraction. As a sketch:

trait Map2Strategy[F[_]] {
  type Lazy[_]
  def map2[A, B](fa: F[A], fb: Lazy[F[B]]): Lazy[F[B]]
}

and then we can either make Lazy[A] = Eval[A] or Lazy[A] = Id[A] depending on the F[_] which maybe we can reflectively select based on if the F[_] is a StackSafeMonad or not.

Or something...

[djspiewak]

I like where this is going. I had a similar thought around the Lazy[_] idea but still trying to piece through how we can make it not-horrible.

[djspiewak]

I wonder if there's a way to make this more general. A => F[B] works extremely well for Kleisli and extremely not-well for almost anything else, since it will force boxing into a Function1 in cases where it just isn't needed.

[johnynek]

Maybe it shouldn't be public at all. I just took what was in Kleisli and made it more particular. I think the current Kleisli hack is adding cost for any Monad that isn't lazy (e.g. Option, or Either), and this function will help there.

[rossabaker]

When in doubt, leave it private. Can always publicize it later if someone finds a second use.

[johnynek]

@djspiewak please take a look and see what you think with your changes merged.