cell: public field is unsafe

use core::cell;

fn main() {
    let x = cell::Cell(Some(~"foo"));
    let y = x.value.get_ref().get_ref();
    do x.with_mut_ref |z| { *z = None; }
    println(*y) // boom!
}
auto merge of #6003 : thestinger/rust/cell, r=nikomatsakis

```rust
use core::cell;

fn main() {
    let x = cell::Cell(Some(~"foo"));
    let y = x.value.get_ref().get_ref();
    do x.with_mut_ref |z| { *z = None; }
    println(*y) // boom!
}
```