Soundness hole in Ptr::read_unaligned

[joshlf]

From what I can tell, it's not exercised anywhere in the codebase.

Ptr::read_unaligned permits reading from a shared pointer through UnsafeCells:

zerocopy/src/pointer/mod.rs

Lines 35 to 53 in 7349fe0

	impl<'a, T, Aliasing, Alignment> MaybeAligned<'a, T, Aliasing, Alignment>
	where
	T: 'a + ?Sized,
	Aliasing: invariant::Aliasing + invariant::AtLeast<invariant::Shared>,
	Alignment: invariant::Alignment,
	{
	/// Reads the value from `MaybeAligned`.
	#[must_use]
	#[inline]
	pub fn read_unaligned(self) -> T
	where
	T: Copy,
	{
	let raw = self.as_non_null().as_ptr();
	// SAFETY: By invariant on `MaybeAligned`, `raw` contains
	// validly-initialized data for `T`. The value is safe to read and
	// return, because `T` is copy.
	unsafe { core::ptr::read_unaligned(raw) }
	}

We should either add an Immutable bound or an AliasingSafe bound.