Doc tests are temporarily disabled until next release cycle, since
current beta Cargo errors on them. Upgrade should be smooth as the
relevant tests are already fixed in this commit.

to get more reproducible output.

Fixes: #32555

This change is aimed at improving cross-crate (inlined) notation of generic
closures. The change modifies `simplify::where_predicates` to handle
parenthesized notation as well as starting to handle supertrait bounds as well.
This was necessary because all output constraints of closures are bound to
`FnOnce` but most trait bounds are that of `FnMut`.

Close #21801

Add a custom module to rustdoc which simplifies the output of `middle::ty` into
a more readable form which tends to be written down anyway!

Closes #20646