Refcounted hooks

[udoprog]

This accomplishes the following:

• Public items are correctly exported if they're within a recursively public module.
• #[test] functions are correctly discovered regardless of where they are using a new CompileVisitor::register_meta hook that is called for every item registered (instead of every item used).
• For consistency, the file loader hook is also made to be refcounted.

TODO

On issue this causes is that nested test functions are picked up incorrectly:

fn function() {
    #[test]
    fn test_fn() {
        assert!(true);
    }
}

This runs as:

testing hello::$0::test_fn                       passed
====
Ran 1 tests with 0 failures in 0.002 seconds

Strictly speaking this also exports functions like this if they are pub. This could be fixed by setting up an internal module which prevents it from being public.