Fix spans in error reporting with unicode characters.

[maximebuyse]

Fixes #1099

[maximebuyse]

This fix only works when the unicode characters are on a different line from the error.

But something like:

fn f(x: usize) {
    let mut bar = 0;
    let mut opt = Some(x);
    /*äöü*/opt.take().map(|foo| bar = foo+1);
}

Open this code snippet in the playground

is still wrong

[maximebuyse]

This fix only works when the unicode characters are on a different line from the error.

But something like:

fn f(x: usize) {
    let mut bar = 0;
    let mut opt = Some(x);
    /*äöü*/opt.take().map(|foo| bar = foo+1);
}

Open this code snippet in the playground

is still wrong

I pushed a new version fixing this as well. I introduced a new dependency to take care of this which simplifies and should hopefully be more reliable.

[franziskuskiefer]

I don't like adding dependencies for something small like this.
But we want to surface backend errors nicely as well. So something may be needed anyway. Let's keep this on the radar to see how to deal with it.
But this fixes the issue at hand. Let's get it in.