Remove code that was awaiting a snapshot

* Disallow structural records everywhere
* Remove all #[cfg(stage0)] stuff
* Remove the last deprecated modes in libcore
* Un-xfail a test
auto merge of #5180 : catamorphism/rust/post-snapshot, r=catamorphism

* Disallow structural records everywhere
* Remove all #[cfg(stage0)] stuff
* Remove the last deprecated modes in libcore
* Un-xfail a test