Bad memory leak when mixing role with attributes

Following code causes big memory leak (on my machine 10MB per second).
It happens only if role has some attributes.
Weakening $a does not help.

```
package A;
use Mouse;

package B;
use Mouse::Role;
has "b" => ("is" => "ro",  "isa" => "Int");

package main;
while (1) {
    my  $a = A->new;
    B->meta->apply($a);
}
```
