`freeze(MaybeUninit<T>) -> MaybeUninit<T>` for masked reads

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@lachlansneff

Why is it necessary to deal with MADV_FREE here?

There's some good discussion about this in

"What The Hardware Does" is not What Your Program Does: Uninitialized Memory

This post is about uninitialized memory, but also about the semantics of highly optimized “low-level” languages in general. I will try to convince you that reasoning by “what the hardware does” is inherently flawed when talking about languages such as Rust, C or C++. https://www.ralfj.de/blog/2019/07/14/uninit.html (edit history)

General MaybeUninit::freeze is going to have to deal with everything brought up there.