use a Constraint synonym instead of empty class

src/Diagrams/Core/Style.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstraintKinds       #-}
 {-# LANGUAGE DataKinds             #-}
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE GADTs                 #-}
@@ -105,7 +106,7 @@ import           Linear.Vector
 --   simply guarantees 'Typeable' and 'Semigroup' constraints.  The
 --   'Semigroup' instance for an attribute determines how it will combine
 --   with other attributes of the same type.
-class (Typeable a, Semigroup a) => AttributeClass a
+type AttributeClass a = (Typeable a, Semigroup a)
 
 -- | An existential wrapper type to hold attributes.  Some attributes
 --   are simply inert/static; some are affected by transformations;