each_char conflict in str::StrSlice and io::ReaderUtil

This code fails to compile:

``` rust
use core::str::*;
fn main() {
    let s = ~"hello";
    // Works:
    each_char(s, |_| {true});
    // Fails:
    s.each_char(|_| {true});
    // Fails:
    for s.each_char |_| {}
}
```

`error: failed to find an implementation of trait core::io::Reader for <V2>`
