rustdoc: render `for<>` on old closure lifetimes
rustdoc: render higher-rank trait bounds

Fix #19915
rustdoc: Use unboxed closure sugar

This unfortunately leaves sugaring Fn/FnMut/FnOnce on cross-crate
re-exports for future work.

cc #19909
rustdoc: Use Fn trait sugar for external paths

Fix #19909
auto merge of #19934 : tomjakubowski/rust/rustdoc-unboxed-closures-redux, r=alexcrichton

We render HRTB and the unboxed closure trait sugar (the so-called 
"parenthesized" notation) where appropriate. Also address the new
`for` syntax on the old closures.