Remove support for older CrateMap versions

[fhahn]

This patch removes the code responsible for handling older CrateMap versions (as discussed during #9593). Only the new (safer) layout is supported now.

[alexcrichton]

Yay red diffs! While you're at it, there's also a few other things which I think could get reorganized.

• Could you move the extern static CRATE_MAP business to just inside the non-windows get_crate_map?
• I think you can remove the iter_module_map function b/c it's so simple and natural now

Otherwise, looks good to me, thanks!

[fhahn]

I reorganized the things mentioned above.

[fhahn]

I think while moving extern static CRATE_MAP to the non-windows get_crate_map I forgot to delete a #[cfg(not(windows))] which lead to a failure of the windows build. I've pushed an updated version of the commit