Lazy

Super-simple scoped lazily-initialised objects, because eager evaluation is for dopes. I am frankly shocked that there is no crate that does this and compiles on any current version of Rust.

Serving suggestion:

```rust let lazy = Lazy::new(|| { println!("Doing something complicated!"); 42 });

println!("{}", *lazy); println!("{}", *lazy); println!("{}", *lazy);

/* Prints: * Doing something complicated! * 42 * 42 * 42 */ ```

Currently implemented with RefCell, but it will not panic. This class is not thread-safe, the project I made it for will eventually need it to be though, so I'll probably update it to use something thread-safe eventually.