#4045 replaced TriviasMap with a sorted Vec (bravo @lucab!)

After that change, there's no need to use a Vec<(u32, Comment)> - can change it to Vec<Comment> and move start field into Comment itself. Or, probably even better, make it struct Comment { span: Span, kind: CommentKind } to match AST nodes.

This would feel more natural to me - start was only external to Comment previously as it was key for the map.

Side note: I'm not sure if SortedComments should be a boxed slice rather than a Vec. Possible that some transforms may want to add/remove comments.