Add roles declarations to allow safe coercions (#16)

Author: kl0tl

src/Foreign/Object.purs

@@ -64,6 +64,8 @@ import Unsafe.Coerce (unsafeCoerce)
 -- | `Object a` represents a homogeneous JS Object with values of type `a`.
 foreign import data Object :: Type -> Type
 
+type role Object representational
+
 foreign import _copyST :: forall a b r. a -> ST r b
 
 -- | Convert an immutable Object into a mutable Object

src/Foreign/Object/ST.purs

@@ -24,6 +24,8 @@ import Data.Maybe (Maybe(..))
 -- | that of `Object a`, except that mutation is allowed.
 foreign import data STObject :: Region -> Type -> Type
 
+type role STObject nominal representational
+
 -- | Create a new, empty mutable object
 foreign import new :: forall a r. ST r (STObject r a)