Added functor instance to Command.
Rewrote library with downladForest.
Finished the new crawler library.

Related to #46 and #49.
Updated comments.
Simplified the type of Successor

It now returns [SuccessorNode a e] instead of ([SuccessorNode a e],
[SuccessorNode a e]). As inner nodes have their own constructors,
returning Inners in their own list was superfluous.
Fixed dynamic crawlers in Crawlers.Library.

Only their outputs were made dynamic, not their inputs.
Added Collection-instance for lists.
Deleted unused types.
Revert "Deleted unused types."

This reverts commit e721a17b8622a07712693ec517ca8b834dd1b9ce.
Deleted unused types.
Closes #49.