Add Parser::Source::Range#line_span

[marcandre]

I've seen about 20 occurrences of range.{first}line..{.}range.last_line or calls to line_range(node) (defined in util.rb)

I feel the name line_span is more descriptive and is less confusing with Source::Buffer#line_range which returns a range of columns for the given line.

I notice that there currently are no extension to parser's classes, but this one seems reasonable.

The result can be particularly useful when dealing with comments, say with each_comment_in_lines of #83

[bbatsov]

I notice that there currently are no extension to parser's classes, but this one seems reasonable.

Agreed.