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

src/librustdoc/rustdoc.rs

@@ -17,6 +17,8 @@
 #[license = "MIT/ASL2"];
 #[crate_type = "lib"];
 
+#[feature(globs, struct_variant)];
+
 extern mod syntax;
 extern mod rustc;
 extern mod extra;