Set unstable feature names appropriately

* `core` - for the core crate
* `hash` - hashing
* `io` - io
* `path` - path
* `alloc` - alloc crate
* `rand` - rand crate
* `collections` - collections crate
* `std_misc` - other parts of std
* `test` - test crate
* `rustc_private` - everything else

src/libcore/raw.rs

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