Data.Functor.FunctorM now works.
Implemented FunctorM for MTree.
Added helper functions in Types.Successor.
Added missing exports.
Partially rewrote Fetch.Forest.

Related to issue #53.
Removed unused functions from Fetch.
Made most of the new fetching logic functional.

The program still doesn't compile. The logic to take into account paths
(Path URL) is still missing.
Mostly integrated paths into Fetch.Forest
Began writing Fetch.Transform. See issue #54.
Added an original filename field to the Failure-type in FetchResult.
See previous commit.
Closes #53.