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
