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/libstd/lib.rs

@@ -484,7 +484,7 @@ pub mod rt;
 // but it may be stabilized long-term. As a result we're exposing a hidden,
 // unstable module so we can get our build working.
 #[doc(hidden)]
-#[unstable(feature = "rand", issue = "0")]
+#[unstable(feature = "rand", issue = "27703")]
 pub mod __rand {
     pub use rand::{thread_rng, ThreadRng, Rng};
 }