Add Box::replace

[mahkoh]

Box currently has no way to replace the contained value while returning the old one.

[dotdash]

Does this really need a method on Box?

let mut b = Box::new(5);
let old = std::mem::replace(&mut *b, 10);

doesn't look that bad.

Edit: Made b mutable.

[mahkoh]

You're right. Didn't think of that one.