Implement reverse arrows for case

[josevalim]

This allows us to track precision inside each branch. For example:

    if x = System.get_env("FOO") do
      x #=> has to be binary
    else
      x #=> has to be nil
    end

• Cache all block constructs (try, receive, cond, with, fn, for)
• Implement nested reverse arrows

[josevalim]

💚 💙 💜 💛 ❤️