Revert "Convert libstd to use the Drop trait"

This reverts commit 28c7a25151.

src/libstd/arc.rs

@@ -217,10 +217,7 @@ fn check_poison(is_mutex: bool, failed: bool) {
 #[doc(hidden)]
 struct PoisonOnFail {
     failed: &mut bool,
-}
-
-impl PoisonOnFail : Drop {
-    fn finalize() {
+    drop {
         /* assert !*self.failed; -- might be false in case of cond.wait() */
         if task::failing() { *self.failed = true; }
     }