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/libfmt_macros/lib.rs

@@ -15,7 +15,7 @@
 //! generated instead.
 
 #![crate_name = "fmt_macros"]
-#![experimental]
+#![unstable]
 #![staged_api]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]