utils: Add CodeMapExt extension trait for span_* methods

[kamalmarhubi]

This commit adds a CodeMapExt extension trait on CodeMap, and moves some
functions to methods there:

• span_after
• span_after_last
• span_before

This better reflects them being lookup methods on the codemap.

[nrc]

I like the refactoring. Is there any need for a trait here? It doesn't seem to make sense for anything other than a CodeMap to implement this stuff, so maybe it is better to just make the methods inherent methods on the CodeMap? If you do think we should keep the trait, I would like a more descriptive name than CodeMapExt CodeMapSpanUtils or something.

[kamalmarhubi]

It doesn't seem to make sense for anything other than a CodeMap to implement this stuff, so maybe it is better to just make the methods inherent methods on the CodeMap?

CodeMap comes from syntex_syntax so I think we have to add things via a trait?

I would like a more descriptive name than CodeMapExt CodeMapSpanUtils or something.

I'm not sure about SpanUtils in the name, as I have a few extras that would end up in here as part of working on the line ranges stuff:

pub struct LineRange {
    pub lo: usize,
    pub hi: usize,
}

pub trait CodeMapExt {
    ...;
    fn lookup_line_range(&self, span: Span) -> LineRange;
}

In my branch where I'm working on that, I actually created a codemap module, but I'm not super happy with that factoring. Very happy to hear suggestions!

[kamalmarhubi]

I guess there's no reason that could't be CodeMapLineRangeUtils or something?

[nrc]

Ok, that makes sense, sorry I got confused with something else.

CodeMapLineRangeUtils sounds good to me.

[kamalmarhubi]

Sounds good. I've updated it to be CodeMapSpanUtils for these methods, and will add another CodeMapLineRangeUtils once that's added.