rustdoc: Add stability dashboard

This commit 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/

Closes #13541
Bug fixes for stability tracking

This commit adds correct stability tracking for struct fields and
corrects some places where rustdoc was not pulling the stability data.
auto merge of #15586 : aturon/rust/stability-dashboard, r=alexcrichton

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