Objects of type T should be implicitely convertible to &T

[Rendered view](https://github.com/Tomaka17/rfcs/blob/master/active/0000-implicit-references.md)
Note that variables passed by `&` can still be modified. http://is.gd/x2XQMn

```rust
use std::cell::Cell;

fn foo(x: &Cell<uint>) { x.set(2) }

fn main() {
    let x = Cell::new(1);
    println!("{}", x); // 1
    foo(&x);
    println!("{}", x); // 2
}
```