Add Church-encoded version of the free foil

[fizruk]

This should be a straightforward adaptation of Control.Monad.Free.Church.
See also «Asymptotic Improvement of Computations over Free Monads»[^1].

[^1] Voigtländer, J. (2008). Asymptotic Improvement of Computations over Free Monads. In: Audebaud, P., Paulin-Mohring, C. (eds) Mathematics of Program Construction. MPC 2008. Lecture Notes in Computer Science, vol 5133. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-70594-9_20