MaybeUninit helpers for rc-box types

[Kixunil]

It'd be nice to provide MaybeUninit helpers like in nightly std (perhaps wait for stabilization). See rust-lang/rust#63291 (comment) and related discussion.

[CAD97]

I don't think I'm going to bother adding any new helpers until std stabilizes them, but I'd be happy to accept and publish them from a PR under an unstable-* feature flag with a prominent "no stability guarantee" disclaimer.

[Kixunil]

That's very reasonable. I will try to do it when I have some free time. :)