rustdoc: Inline documentation across crates

As part of the libstd facade (cc #13851), rustdoc is taught to inline documentation across crate boundaries through the usage of a `pub use` statement. This is done to allow libstd to maintain the facade that it is a standalone library with a defined public interface (allowing us to shuffle around what's underneath it).

A preview is available at http://people.mozilla.org/~acrichton/doc/std/index.html