[WIP] Fix for rust updates

Do not merge.

Fixes #246.

I am getting:

~~~
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:46:45: 46:46 error: cannot move out of captured outer variable
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:46                 *cb.borrow_mut() = Some(box f as Box<Object<Args> + 'static>);
                                                                                                ^
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:23:1: 67:2 note: in expansion of callback!
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:75:5: 83:6 note: expansion site
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:46:45: 46:46 error: cannot move out of captured outer variable
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:46                 *cb.borrow_mut() = Some(box f as Box<Object<Args> + 'static>);
                                                                                                ^
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:23:1: 67:2 note: in expansion of callback!
/Users/brendan/dev/rust/glfw-rs/src/callbacks.rs:92:5: 105:6 note: expansion site
error: aborting due to 2 previous errors
~~~