Additional concept instances for eitherT.

Foldable and monoidA. Also, some documentation corrections.
Trailing comma fix.
Wrong monad instance used.
monoidA instance for maybeT.
Fix of monoidA instance.
Removed monoidA instance for eitherT.

Temporary measure because the compile time constant 'instance' cannot
work correctly until monoidA's structure is changed to be more similar
to e.g. monad.
Additional forgotten forward declarations.
Removed superfluous forward declaration.