Need compiler error for not yet defined functions

[tjpalmer]

We should have a compiler error for the first case here:

$ let f(): Void {
     let g(): Void { h(); }
     g();
     let h(): Void { console.log("hi"); }
   }; f()
interactive#0: fail
$ let f(): Void {
     let g(): Void { h(); }
     let h(): Void { console.log("hi"); }
     g();
   }; f()
hi
interactive#1: void

See also the test case here that I added for working on fixing this issue.

[tjpalmer]

Something we do check already:

$ let f(): Void { let g(): String { a }; let a = "hi"; }
1: { let g(): String { a }; let a = "hi"; }
                       ⇧
[interactive#4:1+34-35]@G: Expected subtype of String, but got Invalid
interactive#4: void
$ let f(): Void { let a = "hi"; let g(): String { a } }
interactive#5: void

Also:

$ let f(): Void { let h = g; let g(): String { "hi" } }
1: f(): Void { let h = g; let g(): String {
                       ⇧
[interactive#1:1+24-25]@T: g__3 is not initialized along branches at [:1+16]
1: let f(): Void { let h = g; let g():
                   ⇧
interactive#1: void

[tjpalmer]

Some additional interesting examples of current behavior:

$ let f(i: Int): Void {
    let g(): Void { h() }
    let h: fn (): Void;
    if (i == 0) {
      h = fn (): Void { console.log("hi") };
    } else {
      h = fn (): Void { console.log("ha") };
    }
  }
2: let g(): Void { h() }
                   ⇧
[interactive#2:2+18-19]@G: Expected function type, but got Invalid
interactive#2: void
$ let f(i: Int): Void {
    let g(): Void { h() }
    let h: fn (): Void = fn (): Void { console.log("hi") };
  }
2: let g(): Void { h() }
                   ⇧
[interactive#3:2+18-19]@G: Expected function type, but got Invalid
interactive#3: void
$ let f(i: Int): Void {
    let h: fn (): Void = fn (): Void { console.log("hi") };
    let g(): Void { h() }
  }
interactive#4: void
$ let f(i: Int): Void {
    let g(): Void { h() }
    let h(): Void { console.log("hi") }
  }
interactive#5: void

[tjpalmer]

I'm ok with restricting forward function references to the cases that work above, but we still want static errors for the fail cases, as reported in the original issue description.