add use-path grammar

[cardoso]

This seems to be missing from #174 . I just took a rough look at wit-parser and this seems to be what it's doing currently.

cc @Mossaka @alexcrichton

[Mossaka]

Thanks! Lgtm

[alexcrichton]

Thanks! Could use-path reuse the preexisting interface production perhaps? That may require renaming interface, however, since include refers to worlds instead of interfaces.

[cardoso]

@alexcrichton can it take the version as well? The parser accepts it, but I didn't see ui tests for it. If the parser is right, I can improve this and add more tests there.

[alexcrichton]

It can, yeah, and would be good to add tests as well!

[cardoso]

Okay, so it seems they're the same:

interface ::= id
            | id ':' id '/' id ('@' valid-semver)?

If this is it, then I'll open the PR for the parser once this is merged.

[cardoso]

It does seem confusing though since it should refer to worlds, but I'll leave it to you.

[cardoso]

Thanks @alexcrichton . I think it reads better now (and uses the same naming as wit-parser).

[lukewagner]

Thanks!