Introduce tidy lint to check for inconsistent tracking issues

This commit
    * Refactors the collect_lib_features function to work in a
      non-checking mode (no bad pointer needed, and list of
      lang features).
    * Introduces checking whether unstable/stable tags for a
      given feature have inconsistent tracking issues.
    * Fixes such inconsistencies throughout the codebase.

src/libfmt_macros/lib.rs

@@ -26,7 +26,7 @@
 #![deny(warnings)]
 
 #![cfg_attr(stage0, feature(staged_api))]
-#![feature(unicode)]
+#![feature(rustc_private)]
 
 pub use self::Piece::*;
 pub use self::Position::*;