Wrote Data.List.Maybe.

Data.List.Maybe provides safe alternatives to those functions in
Data.List which throw errors on empty lists. All safe functions are
re-exported.
Renamed Data.List.Maybe to Data.List.Safe and generalize Maybe to MonadThrow.
Helper.String now uses Data.List.Safe.
Fixed Helper.String not compiling.
Wrote Sysetem.REPL.Command.Nat as a Peano arithmetic helper.
Add comments and ($>) to Helper.Functor.
Fixed import in Helper.Functor.
Wrote System.REPL.Command.Nat.fromNat.
Closes #44.

* Renamed Helper.String.show' to showT.
* The CLI's "unknown" command now takes an arbitrary long list of
arguments.
* System.REPL.paramErr and .checkErr arw now cleaner and deal with both
lower and upper bounds on the number of expected parameters.
* Wrote System.REPL.makeCommandN.