Bad span computations with unicode characters, should be handling them as graphemes

[huonw]

use std::io;

fn main() {
    let s = ~"ZͨA͑ͦ͒͋ͤ͑̚L̄͑͋Ĝͨͥ̿͒̽̈́Oͥ͛ͭ!̏"; while true { break; }
    io::println(s);
}

main.rs:4:46: 4:67 warning: denote infinite loops with loop { ... }, #[warn(while_true)] on by default
main.rs:4     let s = ~"ZͨA͑ͦ͒͋ͤ̚L̄͑͋Ĝͨͥ̿͒̽̈́Oͥ͛ͭ!̏"; while true { break; }
                                                       ^~~~~~~~~~~~~~~~~~~~~

The proper fix for this requires grapheme handling (#7043), e.g. some graphemes are double width.

[huonw]

If #12489 lands, the compiler handles these slightly better, but it's just operating on codepoint counts and assuming they're all single width: i.e. still needs to be changed to work with graphemes.

[pzol]

Visiting for triage, still requires #7043

[alexcrichton]

#7043 has been fixed, so this can actually make progress now!