- Don't make rustc_resolve::Res public; use a new type alias instead

This does *not* currently work for associated items that are
auto-implemented by the compiler (e.g. `never::eq`), because they aren't
present in the source code. I plan to fix this in a follow-up PR.

This caught several bugs where people expected `slice` to link to the
primitive, but it linked to the module instead.

This also uses `cfg_attr(bootstrap)` since the ambiguity only occurs
when compiling with stage 1.

These caused several false positives when documenting rustc, which means
there will likely be many more false positives in the rest of the
ecosystem.