feat: export EntityKeys

The member and associated type were already exported, so this is
strictly an addition. Now, users can express type signatures using this
class, as well as make their own instances.

Closes #90.

Author: pbrisbin

src/Graphula/Key.hs

@@ -6,9 +6,8 @@
 
 -- | Convenience functions for working with 'Key' dependencies
 module Graphula.Key
-  ( onlyKey
-  , keys
-  , Keys
+  ( EntityKeys(..)
+  , onlyKey
   ) where
 
 import Database.Persist