Fix assertions in body of let bindings (#1518)

* Fix assertions in body of `let` bindings

Fixes #1517

* No need to unpack `Binding`

Author: Gabriella439

dhall/src/Dhall/Lint.hs

@@ -44,6 +44,8 @@ removeUnusedBindings _ = Nothing
 fixAsserts :: Expr s a -> Maybe (Expr s a)
 fixAsserts (Let (Binding { value = Equivalent x y, ..}) body) =
     Just (Let (Binding { value = Assert (Equivalent x y), .. }) body)
+fixAsserts (Let binding (Equivalent x y)) =
+    Just (Let binding (Assert (Equivalent x y)))
 fixAsserts _ =
     Nothing