rustdoc: Fix broken struct field search links

Takes the same approach as variants, writes a redirect index page back to the
struct with an anchor to the field in question.

Closes #9524
rustdoc: Generate documentation for foreign items

This slurps up everything inside of an 'extern' block into the enclosing module
in order to document them. The documentation must be on the items themselves,
and they'll show up next to everything else on the module index pages.

Closes #5953
rustdoc: Strip implementations of private traits

Closes #5416
rustdoc: Render stability attributes

Closes #8965
auto merge of #9540 : alexcrichton/rust/more-rustdoc-improvements, r=brson

Commit messages have the details, mostly just knocking out more low-hanging-fruit type issues.