Support use before define within a file

Please allow this:

```
#![test]
fn test_order() {
let _ = assert_eq(identity(u32:0), u32: 0);
()
}

fn identity(x : u32) -> u32 {
x
}
```

Toy examples aside, there are good organizational reasons to allow the developer to order symbols (e.g. functions) within the file in whatever order is best for readability. Succinctly, let's choose 'readable for humans' over 'easy for parser'.