Uses: RWArc<~sprite::Updatable:Freeze+Send> instead of Rc<RefCell<~sprite::Updatable:Send>>
Remove Arc<Tile> in favor of copying `Tile`.
No need for `#[unsafe_destructor]` :-)
(Documentation)
Merge pull request #5 from drbawb/exp/no-refcell

Remove dynamic borrow checking from `src/map.rs`