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
Hah, sorry for the lack of detail, I was in a bit of a hurry last night.

It's not a bug in rustdoc; it's due to two things:

1. The stability summary is build prior to a crate folding step that drops some of the items. I did this because rustdoc *deletes* trait impls, tracking them in a side table instead (so that they show up on the trait and type, not on the module defining them). But for stability summarization, we care about the module where it's defined.

2. rustdoc normally filters out only some of the private items at the top level -- basically, modules. In other cases, it deals with privacy as part of rendering, e.g. showing that a `struct` has some additional private fields.