Feature gates

This crate declares the set of past and present unstable features in the compiler. Feature gate checking itself is done in librustc_ast_passes/feature_gate.rs at the moment.

Features are enabled in programs via the crate-level attributes of #![feature(...)] with a comma-separated list of features.

For the purpose of future feature-tracking, once a feature gate is added, even if it is stabilized or removed, do not remove it. Instead, move the symbol to the accepted or removed modules respectively.