Allow taking ownership of underlying line string

[jmagnuson]

Fields are currently hidden behind ref-getter functions Line::source and Line::line. This was done to allow for flexibility in the underlying data types (especially source which should really sit behind an Arc) without breaking the exposed API. But it would be nice to at least provide a way for reusing the existing String allocated for the line. A few options:

• Straight destructuring would require making the fields pub, which I would personally be okay with, except for the constraint mentioned above. Just line being public could work.

• Line::into_parts -> (PathBuf, String) could work (and just clone the Path if/when it becomes Arc'ed).

• Line::take_line -> String and panic, or return "".to_string on future calls to that or Line::line.

• impl Into<String> for Line and just consume self to pass along line.