Implement feature-gating for the compiler

A few features are now hidden behind various #[feature(...)] directives. These
include struct-like enum variants, glob imports, and macro_rules! invocations.

Closes #9304
Closes #9305
Closes #9306
Closes #9331
rustpkg: Fix fetching remote packages

Closes #9193
auto merge of #9741 : catamorphism/rust/rustpkg-remotes, r=cmr

r? @cmr Closes #9193
Add appropriate #[feature] directives to tests
auto merge of #9703 : alexcrichton/rust/compiler-features, r=cmr

This implements the necessary logic for gating particular features off by default in the compiler. There are a number of issues which have been wanting this form of mechanism, and this initially gates features which we have open issues for.

Additionally, this should unblock #9255