Add Unit data type and instances
Add Show Unit instance
Add unit :: forall a. a -> Unit
Add Unit example
Merge pull request #406 from puffnfresh/unit

Add Unit data type and instances
Make `unit` a value
Merge pull request #407 from garyb/unit

Make `unit` a value
Fix compile warnings. Remove unnecessary function in entails.
Specify haskeline dependency, fix #412