SumBag: Don't throw away whole subtrees when deleting

This was a bug in the `delmin` helper that is used to delete the smallest
element of the right tree. It deleted too much.

src/Data/SumBag.hs

@@ -150,7 +150,7 @@ delOnce e = case snd (NE.uncons (multiplicity e)) of
 
 delmin :: Monoid a => Tree a -> (Element a, Tree a)
 delmin Leaf = error "delmin: Empty tree"
-delmin (Node _ e Leaf _) = (e, Leaf)
+delmin (Node _ e Leaf right) = (e, right)
 delmin (Node _ e left right) = (\left' -> balance1 e left' right) <$> delmin left
 
 balanceBound :: Int