Create abstraction over paths for use in librustc_resolve

[Manishearth]

Currently, librustc_resolve passes around paths as &[Ident].

Ideally, we'd have an abstraction over paths that:

• contains this slice
• contains a source node id for linting
• contains a flag for whether or not the path is speculative -- sometimes we try to resolve paths not specified by the user to produce better error messages. We shouldn't be throwing errors for these.
• eventually (out of scope for this change) -- contains a flag for whether or not the flag was originally a global path (::foo)

Currently the last two are kinda compressed into an Option<NodeId> argument to resolve_path -- when it's speculative, the Option is None.

Having a Path abstraction would be nice.

[IsaacWoods]

If nobody is working on this, I'd like to give it a go to get my feet wet (never contributed to rustc tho)?

[TheSirC]

Is there anymore work to be done on that end ? @Aaronepower ? @IsaacWoods ? @Manishearth ?

[petrochenkov]

I'll go ahead an close this as not-an-issue.
Interfaces in librustc_resolve generally need to be rethought into something more systematic, especially after they changed a lot recently to accommodate new features, but simply grouping parameters of resolve_path into a struct won't move things into the right direction, but mostly bring unnecessary churn.