Add support for recursion

[emilsoman]

The following should type check successfully:

    fn a : Loop(Int | String) do
      if true do
        b()
      else
        123
      end
    end

    fn b : Loop(Int | String) do
      if true do
        c()
      else
        "abc"
      end
    end

    fn c : Loop(Int | String) do
      a()
    end

    fn d : Int | String do
      a()
    end

I noticed there was no issue for this, but @polvalente is already working on this in #87 so just adding it here.