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/array.rs

@@ -12,7 +12,7 @@
 //! up to a certain length. Eventually we should able to generalize
 //! to all lengths.
 
-#![experimental] // not yet reviewed
+#![unstable] // not yet reviewed
 
 use clone::Clone;
 use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};