Fix: MemoryRng not working when used by mut ref

src/rng.rs

@@ -199,4 +199,11 @@ where
     {
         (**self).next_bytes(f)
     }
+
+    fn draw_alpha<F: Field>(&mut self) -> F {
+        (**self).draw_alpha()
+    }
+    fn draw_positions(&mut self, count: usize, domain_size: usize) -> Vec<usize> {
+        (**self).draw_positions(count, domain_size)
+    }
 }