Start small. If it works well we can increase usage bit by bit as time passes.

My main motivation for doing this is to get rid of the + 1 I otherwise have to add in #150201 on this line:

                crate::directives::line::line_directive(file, zero_based_line_no + 1, &line)

But I think this is a nice general improvement by itself.

Note that we keep using "0" to represent "no specific line" because changing to Option<LineNumber> everywhere is a very noisy and significant change. That can be changed later if wanted, but let's not do it now.