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
testsuite: Re-xfail record-trailing-comma

It was xfailed before the other commits in this pull request,
so no big deal. I'll look into it later.
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