Expose more ways for users to manipulate Span

[tangmi]

I often find myself reaching for pest::span::Span to keep a mapping back to the parsed source file. I sometimes run into issues because Span::input is not pub.

I'd like to propose adding two methods to Span to give it more flexibility for users:

• pub fn sub_span(&self, range: impl std::ops::RangeBounds<usize>) -> Option<Span>
  • Allows one to sub-slice a Span, returning a smaller span (or "substring") which preserves the original Span's input.
• pub fn lines_span(&self) -> LinesSpan
  • where LinesSpan is an iterator, much like pest::span::Lines, but returns Span instead of &str.
  • Lines can be implemented as span.lines_span().map(|span| span.start_pos().line_of())

I've created a draft PR with a more concrete example of this proposal: #456.

Thanks for your time!