core: Add unwrap()/unwrap_err() methods to Result

These implementations must live in libstd right now because the fmt module has
not been migrated yet. This will occur in a later PR.

Just to be clear, there are new extension traits, but they are not necessary
once the std::fmt module has migrated to libcore, which is a planned migration
in the future.

src/libstd/rt/task.rs

@@ -436,7 +436,7 @@ mod test {
     #[test]
     fn rng() {
         use rand::{StdRng, Rng};
-        let mut r = StdRng::new().unwrap();
+        let mut r = StdRng::new().ok().unwrap();
         let _ = r.next_u32();
     }