Handle destructors properly in conc::Atomic.

Previously, they simple werent being run.

conc/src/atomic.rs

@@ -279,6 +279,20 @@ impl<T> Default for Atomic<T> {
     }
 }
 
+impl<T> Drop for Atomic<T> {
+    fn drop(&mut self) {
+        // We use the neat `get_mut` to get around the overhead of atomics.
+        let ptr = self.inner.get_mut();
+
+        if !ptr.is_null() {
+            // As the read pointer was not null, we can safely call its destructor.
+            unsafe {
+                drop(Box::from_raw(ptr));
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

conc/src/sync/treiber.rs

@@ -123,6 +123,8 @@ mod tests {
         assert_eq!(*stack.pop().unwrap(), 200);
         assert_eq!(*stack.pop().unwrap(), 1);
         assert!(stack.pop().is_none());
+
+        ::gc();
     }
 
     #[test]
@@ -146,6 +148,8 @@ mod tests {
             assert!(stack.pop().is_none());
             assert!(stack.pop().is_none());
         }
+
+        ::gc();
     }
 
     #[test]