Instead of forcibly always aborting compilation, allow usage of
#[warn(unknown_features)] and related lint attributes to selectively abort
compilation. By default, this lint is deny.
A few features are now hidden behind various #[feature(...)] directives. These
include struct-like enum variants, glob imports, and macro_rules! invocations.
Closes#9304Closes#9305Closes#9306Closes#9331
