Add functional dependencies

[ivenmarquardt]

Functional dependencies presuppose multi-parameter type classes and are required to implement resurcion schemes without lots of boilerplate. Consequently, scriptum won't provide recursion schemes until version 2.

However, this doesn't mean that appropiate types doesn't implement cata-/paramorphisms and ana-/apomorphisms respectively. Just without the recursion scheme machinery and thus with lots of repetition.