libcore: Replace a bunch of "== None" with ".is_none()".

Generally, "== None" calls into the shape glue, and it's also more useful.

src/libcore/rand.rs

@@ -408,7 +408,7 @@ mod tests {
     #[test]
     fn choose_option() {
         let r = rand::Rng();
-        assert r.choose_option(~[]) == None::<int>;
+        assert r.choose_option(~[]).is_none();
         assert r.choose_option(~[1, 1, 1]) == Some(1);
     }
 
@@ -431,7 +431,7 @@ mod tests {
             {weight: 0u, item: 42},
             {weight: 1u, item: 43}
         ]) == Some(43);
-        assert r.choose_weighted_option(~[]) == None::<int>;
+        assert r.choose_weighted_option(~[]).is_none();
     }
 
     #[test]