`Box::as(_mut)_ptr` is unsound, `alloc` feature does not support msrv

[bluurryy]

The current implementation of as_mut_ptr() looks like this

// This is a primitive deref, not going through `DerefMut`, and therefore not materializing
// any references.
&raw mut **b

What the comment says is true for the standard library type, but allocator_api2's Box has no primitive deref and it will go through DerefMut.

This causes a stacked borrows violation when running as_ptr's doc test in miri.

The implementation can be replaced by

b.0.as_ptr()

This has the side effect that the alloc feature becomes compatible with the msrv.

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I adapted Box's doc tests so they actually test allocator_api2::boxed::Box and they all pass miri[^1] with that change. Would you be interested in a PR that makes those changes to the doc tests?

[^1]: With MIRIFLAGS=-Zmiri-ignore-leaks, because some tests intentionally leak