Auto merge of #161113 - JonathanBrouwer:rollup-SBl74tq, r=JonathanBrouwer

Rollup of 6 pull requests

Successful merges:

 - rust-lang/rust#159963 (core: implement FusedIterator for StepBy)
 - rust-lang/rust#152826 (tests/ui/union: add annotations for reference rules)
 - rust-lang/rust#160556 (cargotest: replace Servo with standalone Stylo)
 - rust-lang/rust#161011 (tests/ui/rust-2018: add annotations for reference rules)
 - rust-lang/rust#161083 (Revert "Add regression test for const fn returning Vec of boxed dyn Any")
 - rust-lang/rust#161112 (rustc.1: `-O` is `-C opt-level=3`)