By creating a *mut [T] directly (without going through &mut [T]), avoid
questions of validity of the contents of the slice.

Consider the following risky code:

```rust
unsafe {
    let mut v = Vec::<bool>::with_capacity(16);
    v.set_len(16);
}
```

The intention is that with this change, the above snippet will be
sound because Vec::drop does no longer produces a mutable slice of
the vector's contents.

Update Vec drop with a comment to explain why we want to use a raw
slice, and extend this pattern to also include the Vec's IntoIter.