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 (e.g., I should write a struct MoveItems and appropriate implementation for BinaryHeap instead), 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!
This is one of those subtle test things. Technically, if your iterator impl is totally busted, and the loop never runs at all, this test will pass. I recommend making `iterout` a `vec`, and `.collect()`ing the `into_iter` into a Vec. Then just check if the two are equal.