allow recursive object methods

It's a bit obnoxious when writing an object type that can return instances of itself that we need a tag to break up the recursion:

```
tag foo_tag = foo;

type foo = obj {
    fn get_children() -> [foo_tag];
};

obj new_foo() {
  fn get_children() -> [foo_tag] { [] }
}
```

It'd be much nicer if rust would just let us write `fn get_children() -> [foo]`.
