For loop iteration allows mutating container.

Following code passes borrow checking leading to serious memory unsafety.

``` rust
fn main() {
    let mut v = Vec::new();
    for _ in v.iter() {
        v.push('j');
    }
}
```

Code compiles with `&c` instead of `_` too. Loop fails to compile only in `for c in v.iter()` case where there's an explicit variable with reference or when the loop is desugared manually.
