Improve backward compat. Add nullable() shortcut

* List validation is now identical to that of v0.12 (using natural notation);
* The `optional(x)` shortcut now only allows `MISSING` values along with `x`;
* The `nullable(x)` shortcut allows `None` along with `x` (in v0.12 it was
  the job of `optional()` as it did not distinguish between `None` and
  `MISSING`).