message: tests.elle.meta: Add messages to some static assertions.
diff: @@ -11,8 +11,9 @@ namespace list
  {
  using l1 = List<int, float, double>;
  using l2 = List<int, float, double, l1>;
  - static_assert(l1::size == 3);
  - static_assert(l1::size - l2::size == -1);
  + static_assert(l1::size == 3, "l1::size should be 3");
  + static_assert(l1::size - l2::size == -1,
  + "Difference of sizes should be -1");
  }
  namespace apply
  