Beginning-of-line detection in pretty-printer isn't reliable

It only recognizes 'hard' line breaks. Spaces might also cause lines to be broken. This program ends up with an extra blank line above the comment because of this:

``` rust
fn main() {
    let x = Foo {x: 10, y: 10000000000,
             // This a comment
             z: 45000000000000000,
             q: 2444444444444444};
}

```
