Impls Finalize, Trace for PhantomData and Rc<[T]>

[remexre]

I knew I forgot something in #58...

Also fixes a typo.

[remexre]

Updated to merge cleanly against 0.4.1, added an Rc<[T]> impl too, since that's missing.

[andersk]

This Rc<[T]> impl is another instance of #134.

use gc::Gc;
use std::rc::Rc;

fn main() {
    let r: Rc<[Gc<()>]> = Rc::new([Gc::new(())]);
    Gc::new((Rc::clone(&r), r));
    // → thread 'main' panicked at 'Can't double-unroot a Gc<T>'
}

• Remove Trace implementation for Rc #153