`DeferMut for Imp` is unsound

This simple code produces undefined behavior without unsafe code involved.

```rust
use interior_mutability_pointer::Imp;

fn main() {
    let mut a = Imp::new(vec![1]);
    let mut b = a.clone();

    let x = &mut a[0];
    b.clear();
    b.shrink_to_fit();

    println!("{}", *x); // USE AFTER FREE
}
```