TODO: Support for functions with none return type

[AjaniBilby]

No description provided.

[AjaniBilby]

This has already been implemented, I think during #11, but not entirely sure when I did it.
But this does compile so... 🤷

fn main(): none {
  return;
}

(module
  (type (;0;) (func))
  (func (;0;) (type 0)
    return
    unreachable)
  (memory (;0;) 1 1)
  (global (;0;) (mut i32) (i32.const 0))
  (export "_start" (func 0))
  (export "main" (func 0)))