rustdoc: Linkify all reexports.

This way each component of a reexport path is click-able to the destination that
it's referencing.
rustdoc: Start rendering variants (redirect to enum)

This allows reasonable behavior when an enum is clicked in an import.
rustdoc: Highlight function names

Closes #9460
rustdoc: Strip hidden docs by default.
rustdoc: Reduce ambiguity with clean::Type

The "Unresolved" variant could be completely removed becuase it turns out that
the interim state only very briefly lives.
rustdoc: Collapse before unindenting

Doesn't make much sense to unindent each line individually and *then* collapse
them all together.
rustdoc: Fix an unindentation bug when collapsing

Turns out eagerly trimming comes back to bite you :(