mtl-2.2.1: Monad classes, using functional dependencies

Monad classes using functional dependencies, with instances for various monad transformers, inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, by Mark P Jones, in Advanced School of Functional Programming, 1995 (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html).