New lesson on polymorphic variants

[dbuenzli]

@jakubsvec001

Several OCaml topics can be taught by example with tangential reference to type-theoretic terminology or implementation details, but polymorphic variants and GADTs in particular can require pointed reference to their underpinnings to provide an accurate model for how they work, which introduces additional complexity and difficulty to the lesson.

Are you really sure ? The manual proposes a short, to the point and beginner friendly introduction to polymorphic variants, I’m not sure its worth adding much more.