VecDeque: fix for stacked borrows
`VecDeque` violates a version of stacked borrows where creating a shared reference is not enough to make a location *mutably accessible* from raw pointers (and I think that is the version we want). There are two problems:
* Creating a `NonNull<T>` from `&mut T` goes through `&T` (inferred for a `_`), then `*const T`, then `NonNull<T>`. That means in this stricter version of Stacked Borrows, we cannot actually write to such a `NonNull` because it was created from a shared reference! This PR fixes that by going from `&mut T` to `*mut T` to `*const T`.
* `VecDeque::drain` creates the `Drain` struct by *first* creating a `NonNull` from `self` (which is an `&mut VecDeque`), and *then* calling `self.buffer_as_mut_slice()`. The latter reborrows `self`, asserting that `self` is currently the unique pointer to access this `VecDeque`, and hence invalidating the `NonNull` that was created earlier. This PR fixes that by instead using `self.buffer_as_slice()`, which only performs read accesses and creates only shared references, meaning the raw pointer (`NonNull`) remains valid.
It is possible that other methods on `VecDeque` do something similar, miri's test coverage of `VecDeque` is sparse to say the least.
Cc @nikomatsakis @Gankro
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