std: Deprecate the thunk module

This has been replaced with `FnBox`

src/libstd/thunk.rs

@@ -11,6 +11,7 @@
 // Because this module is temporary...
 #![allow(missing_docs)]
 #![unstable(feature = "thunk")]
+#![deprecated(since = "1.2.0", reason = "use FnBox instead")]
 
 use alloc::boxed::{Box, FnBox};
 use core::marker::Send;