Eval refactor, fixing bugs in evaluation order

[kristerr]

This PR wouldn't be possible without help of @MateuszKubuszok. Thank you!

Also, closes: #189, #204, #205.

[MateuszKubuszok]

You might want to do it in another PR (and perhaps need a new major version) but since Sealed no longer requires Eval to work, you can implement #168 much easier: you only need to define some:

trait Monad[F[_]] {
  def pure[A](a: A): F[A]
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B]
  def tailRecM[A, B](a: A)(f: A => F[Either[A, B]): F[B]
}

required for eval and then create an instance out of Cats' Monad instance as an integration.