Abstract review file contents

[danobi]

Right now we pass around strings and string references everywhere. It mostly works, except in a few places it's ambiguous whether or not text is quoted or unquoted text that represents quoted text (or some other combination). It would be great to unify all text behind a data structure ike LineType:

prr/src/review.rs

Lines 51 to 59 in 6b4a4bf

	/// Represents a single line in a review file.
	enum LineType<'a> {
	/// Original text (but stored without the leading `> `)
	Quoted(&'a str),
	/// A snip (`[..]`)
	Snip,
	/// User supplied comment
	Comment(&'a str),
	}

That way, we can unambiguously pass around Vec<LineType> (or similar) and unify all the quoting / dequoting.