Support OMG-WTF-8 in wtf8.rs.

Author: kennytm

src/libstd/lib.rs

@@ -285,6 +285,7 @@
 #![feature(panic_unwind)]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
+#![feature(ptr_offset_from)]
 #![feature(raw)]
 #![feature(renamed_spin_loop)]
 #![feature(rustc_attrs)]