Links in documentation largely broken

[hannobraun]

Many of the links in the generated API documentation are largely broken. This is not something we can fix easily: The only viable way to link to other resources within the documentation are relative links. In the presence of re-exports, the relative link is only going to be correct for either the original or the re-exported type.

I think we can use intra-doc links to fix this. This feature is currently on its way to being stabilized: rust-lang/rust#43466, rust-lang/rust#74430

[hannobraun]

Intra-doc links are stable now. Removing the upstream label.