rustdoc: Correctly name lifetimes in bounds

Fix #16518
rustdoc: Support unboxed fn sugar in bounds
rustdoc: Remove dummy UnknownBound variant
rustdoc: Include lifetimes in re-exported bounds

Fix #17818
auto merge of #17798 : tomjakubowski/rust/rustdoc-fix-bounds, r=alexcrichton

This PR adds support in rustdoc for properly naming lifetimes in bounds, instead of just showing `'static` for everything. It also adds support for unboxed function sugar bounds, which were also previously rendered as `'static`.