Unicode combining characters cause unnecessary warnings

[euclio]

On formatting the following code with rustfmt:

fn main() {
    println!("{}", "क्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षि");
}

It formats it as:

fn main() {
    println!("{}",
             "क्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षिक्षि");
}

but outputs: src/lib.rs:3: line exceeded maximum length (sorry), when the line is clearly shorter than the threshold.

I think the problem lies in that rustfmt uses len_utf8() to calculate line length. It should probably use unicode grapheme clusters, but the underlying string buffer type doesn't support this.

[nrc]

Yeah, this is annoying, sorry.

Dup of #6