rustdoc: add stability dashboard

This PR adds a crate-level dashboard summarizing the stability levels of all items for all submodules of the crate.

The information is also written as a json file, intended for consumption by pages like http://huonw.github.io/isrustfastyet/

Along the way, fixes a few bugs in stability tracking and places where rustdoc was not pulling the existing stability data.

Closes #13541
I would have suspected that the previous passes of rustdoc would have stripped non-public items. Did you find that to not be the case?