Package debug libstd with binaries

[brson]

For debugging it's useful to have a debug build of std. Figuring out how to make the debugger actually locate the source in all scenarios could be complicated.

[brson]

There are some options: we could just provide separate debuginfo for the optimized std, but we could also provide an unoptimized std with debuginfo.

[alexcrichton]

I would personally be very worried about binary distribution size if we distribute yet another set of copies of the standard library, but the idea here sounds great!

[brson]

Still something we want to do.

[retep998]

I'd very much like an optional package that bundles .pdb debug info for the exe and dll binaries in the nightlies. Sometimes I want to debug what rustc is doing or profile it, and I can't really do it unless I build rustc from source.

[steveklabnik]

Triage: no changes here. I wonder if we'll do this, or if we'll eventually get to "cargo aware std", where you can get your own debug build when building your project.