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/sync/mpsc/mpsc_queue.rs

@@ -35,7 +35,7 @@
 //! method, and see the method for more information about it. Due to this
 //! caveat, this queue may not be appropriate for all use-cases.
 
-#![experimental]
+#![unstable]
 
 // http://www.1024cores.net/home/lock-free-algorithms
 //                         /queues/non-intrusive-mpsc-node-based-queue

src/libstd/sync/mpsc/select.rs

@@ -46,7 +46,7 @@
 //! ```
 
 #![allow(dead_code)]
-#![experimental = "This implementation, while likely sufficient, is unsafe and \
+#![unstable = "This implementation, while likely sufficient, is unsafe and \
                    likely to be error prone. At some point in the future this \
                    module will likely be replaced, and it is currently \
                    unknown how much API breakage that will cause. The ability \

src/libstd/sync/mpsc/spsc_queue.rs

@@ -33,7 +33,7 @@
 //! concurrently between two tasks. This data structure is safe to use and
 //! enforces the semantics that there is one pusher and one popper.
 
-#![experimental]
+#![unstable]
 
 use core::prelude::*;