`fmt::Debug` should not escape printable characters

(imported from improperly closed bug #22412)

``` rust
pub fn main() {
    println!("{}", "äöüß");
    println!("{:?}", "äöüß");
}
```

prints:

```
äöüß
"\u{e4}\u{f6}\u{fc}\u{df}"
```

but probably should print:

```
äöüß
"äöüß"
```

(See discussion on #22412 for strategies for fixing this; team seemed pretty much on board for this change.)
