SumBag: Make datatype strict

This means that `insert undefined bag` now evaluates to undefined in WHNF.

src/Data/SumBag.hs

 {-# LANGUAGE RoleAnnotations #-}
+{-# LANGUAGE StrictData #-}
 
 -- | == Multisets with constant-time fold.
 --

tests/Data/SumBagSpec.hs

@@ -34,6 +34,10 @@ insertSpec = describe "insert" $ do
     let bag = iterate (SumBag.insert (si 1)) SumBag.empty !! 10
     in SumBag.toAscList bag `shouldBe` (replicate 10 (si 1))
 
+  it "is strict" $
+    let bag = SumBag.insert @(Sum Int) (error "") SumBag.empty
+    in seq bag (return ()) `shouldThrow` anyErrorCall
+
 
 deleteSpec :: Spec
 deleteSpec = describe "delete" $ do