constraints:

- clean/inline.rs needs this map to fill in traits when inlining
- fold.rs needs this map to allow passes to fold trait items
- html/render.rs needs this map to seed the Cache.traits map of all
  known traits

The first two are the real problem, since `DocFolder` only operates on
`clean::Crate` but `clean/inline.rs` only sees the `DocContext`. The
introduction of early passes means that these two now exist at the same
time, so they need to share ownership of the map. Even better, the use
of `Crate` in a rustc thread pool means that it needs to be Sync, so it
can't use `Lrc<Lock>` to manually activate thread-safety.

`parking_lot` is reused from elsewhere in the tree to allow use of its
`ReentrantMutex`, as the relevant parts of rustdoc are still
single-threaded and this allows for easier use in that context.

Fixes #53812

A type's fully qualified name will now wrap once it gets to the
`[-][src]` link aligned against the right edge of the content area.
Previously the two would overlap and the name would only wrap when
hitting the edge of the content area. The `[-][src]` link will main at
the top of the header.

Fix trait item doc setting, add new setting, start hiding elements by default and then showing them up

Removes some unused code and de-publicizes structs

Previously these functions relied on TLS but we can instead thread the
relevant state through explicitly.

Removes dependency on UnstableFeatures from markdown rendering