Replace use detection with a call graph

[giacomocavalieri]

I noticed that a private recursive function is not marked as unused. For example this one raises no warnings:

fn wobble() {
  wobble()
}

[lpil]

Yup! The current system isn't very good. I've been wanting to replace it with a proper call graph for ages but I've not got round to that yet.

I've not made an issue for it either, so let's use this one.

[giacomocavalieri]

Cool! Do you already have an idea on how to do that? I'm really curious to learn how that's going to be implemented 👀

[lpil]

Closes https://github.com/gleam-lang/gleam/issues/2579

[giacomocavalieri]

In this test the unused variable should be marked as unused!

pub fn loop(cond: Bool, unused: Int) {
  case cond {
    True -> loop(False, unused)
    False -> 1
  }
}