Fix panic and generation issue in Arena::drain()

[taiki-e]

This fixes the second and fourth issues of #30.

2. If you add some elements and then use Arena::drain(), again, this should bump the generation and it doesn't, which is a similar bug.

4. drain() doesn't fix up the free list or other state. After you drain the whole arena, is still has a positive len(). Then you'll get asserts when you try to use the arena. I'm not sure what exactly will be involved to fix this.

[fitzgen]

Thanks!