Implement into_iter() for BinaryHeap.

Whilst browsing the source for BinaryHeap, I saw a FIXME for implementing into_iter.  I think, since the BinaryHeap is represented internally using just a Vec, just calling into_iter() on the BinaryHeap's data should be sufficient to do what we want here.  If this actually isn't the right approach, let me know and I'll happily rework this.

Both of the tests that I have added pass.  This is my first contribution to Rust, so please let me know any ways I can improve this PR!