rustc: Forbid plugin_registrar in only rlib form

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
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 #15565 : alexcrichton/rust/issue-15475, r=huonw

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
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
collections: Move push/pop to MutableSeq

Implement for Vec, DList, RingBuf. Add MutableSeq to the prelude.

Since the collections traits are in the prelude most consumers of
these methods will continue to work without change.

[breaking-change]
collections: Deprecate push_back/pop_back
Convert some push_back users to push
collections: Move push/pop docs to MutableSeq
vim: Add MutableSeq
collections: Make push_back/pop_back default methods