Improvements to feature staging

This gets rid of the 'experimental' level, removes the non-staged_api
case (i.e. stability levels for out-of-tree crates), and lets the
staged_api attributes use 'unstable' and 'deprecated' lints.

This makes the transition period to the full feature staging design
a bit nicer.

src/libstd/dynamic_lib.rs

@@ -12,7 +12,7 @@
 //!
 //! A simple wrapper over the platform's dynamic library facilities
 
-#![experimental]
+#![unstable]
 #![allow(missing_docs)]
 
 use prelude::v1::*;