rustdoc: incorporate stability index throughout

This commit hooks rustdoc into the stability index infrastructure in two
ways:

1. It looks up stability levels via the index, rather than by manual
attributes.

2. It adds stability level information throughout rustdoc output, rather
than just at the top header. In particular, a stability color (with
mouseover text) appears next to essentially every item that appears
in rustdoc's HTML output.

Along the way, the stability index code has been lightly refactored.