`fmt::Debug` should not escape printable characters

[pnkfelix]

(imported from improperly closed bug #22412)

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.)

[steveklabnik]

Triage: no change. I do wonder about this kind of change with regards to stability though.

[estebank]

I see where the sentiment comes from, but do consider that there're multiple ways to encode the same visual output. Debug using the \u{} format for those characters helps differentiate easily between ö and o¨, for example.

[bluss]

This was fixed in #34485 and the examples in the original report have the desired output.