Add the standard monad transformers

[bkc39]

At the bare minimum, we need

• [x] OptionT
• [x] ErrorT
• [x] ReaderT
• [x] WriterT
• [x] StateT
• [x] ContT

There are already full implementations of these in ocaml-monadic. It's just a matter of porting.

However, it may be better to consider modeling off of the haskell mtl.

[bkc39]

All of the implementations are here now. What's left is defining the abstract signature for the transformer and having each of the Transformers include a common sig, which will essentially be

module type TRANSFORMER = sig
  include Monad.S
  val lift : 'a M.t -> 'a t
end

The problem here is that M is unbound so you have to make it a functor. The problem with that is that it precludes having the "right" signatures for the higher kinded transformers like ReaderT. This means we have to

• [x] #7 Implement the abstract TRANSFORMER signature.
• [ ] Hide the implementations of each of the concrete transformers. Ensure that the MONAD_DEF instance is passed first into the TRANSFORMER functor for higher kinded TRANSFORMERs.