Prevent calling `collect` multiple times

[giann]

Most of the time collect will clear external resources and should not be called more than once. Currently, we avoid this with flag:

object File {
    collected: bool = false,

    // ...

    mut fun collect() > void {
        if (!this.collected) {
            this.file.close();
            this.collected = true;
        }
    }    

    // ...
}

But this is not ideal, namely because nothing prevents collected to be modified by the user.

Buzz should itself enforce the fact that that special method is called once.