Can't cast `char` to `char`

All other types can be cast to themselves just fine, but trying to compile

``` rust
fn main() {
    let a = 'c';
    let b = a as char;
}
```

gives

```
bug.rs:3:13: 3:22 error: only `u8` can be cast as `char`, not `char`
bug.rs:3     let b = a as char;
                     ^~~~~~~~~
error: aborting due to previous error
```
