Feature request: `pop` arbitrary entry.

When attempting to implement a pool with slab, we have a need to access/pop an arbitrary element. I propose the api
```rust
impl<T> Slab<T>{
  pub fn take(&mut self)->Option<T>{
    ...
  }
}
```

Which will remove and return an arbitrary element (if one exists). As for implementation, it seems simple enough to always remove and return the element at `self.next - 1`