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/libcore/raw.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 #![allow(missing_docs)]
-#![experimental]
+#![unstable]
 
 //! Contains struct definitions for the layout of compiler built-in types.
 //!