Add `NonNull` convenience methods to `Box` and `Vec`
Implements the ACP: https://github.com/rust-lang/libs-team/issues/418.
The docs for the added methods are mostly copied from the existing methods that use raw pointers instead of `NonNull`.
I'm new to this "contributing to rustc" thing, so I'm sorry if I did something wrong. In particular, I don't know what the process is for creating a new unstable feature. Please advise me if I should do something. Thank you.
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