Control.App: use new type instead of Void as marker for I/O
Control.App: use new type instead of Void as marker for I/O

Took me some time to figure out that `Void` means `IO`.

Express that knowledge more explicitly.

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm