Implement Reflect trait with a variant on the standard OIBIT

semantics that tests the *interface* of trait objects, rather
than what they close over.

src/libcore/lib.rs

@@ -72,6 +72,7 @@
 #![feature(rustc_attrs)]
 #![feature(optin_builtin_traits)]
 #![feature(concat_idents)]
+#![feature(reflect)]
 
 #[macro_use]
 mod macros;