Add support for private functions

[rtulip]

It'd be great to be able to mark functions as pub to allow them for use outside of the file they've been written in.

// in foo.hay
fn foo_x() {}
pub fn foo_y() {}

// in main.hay
include "foo.hay"
fn main() {
    // foo_x -- Error: Function `foo_x` is private
    foo_y // this is fine
}