Add support to span

[celinval]

Users should be able to retrieve the code span of SMIR items.

[ouz-a]

@rustbot claim

[oli-obk]

how should we expose it. If we expose it directly (have PathBuf, line/col info and expansion info directly in the smir::Span struct), that may get expensive. If we expose it indirectly, we can do it like .kind() on smir::Ty, computing it only when requested and otherwise just storing an index.

[celinval]

We should encapsulate the path, so no one relies on having direct access to them. We probably want to intern the pathbuf for the file, so we don't keep replicating it.

[ouz-a]

Should we return rustc_span::Span or stable_mir::Span, agree with oli here exposing indirectly will suffice for most needs.

[ouz-a]

can we close this now ?

[oli-obk]

Right now the spans are opaque and thus not very useful. We should add a way to retrieve file, line, col and expansion information.

[celinval]

I believe we can close this now, right? Feel free to reopen if there's more work to be done here. Thanks!