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/path/mod.rs

@@ -59,7 +59,7 @@
 //! println!("path exists: {}", path.exists());
 //! ```
 
-#![experimental]
+#![unstable]
 
 use core::marker::Sized;
 use ffi::CString;