Add support for rust::Option<Box<T>>

[djmitche]

While general support for Option<T> is difficult, Option<Box<T>> has a well-known layout due to the NPO, so we can replicate that layout in C++.

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This is a very early draft!

[djmitche]

I'd like a little feedback here. I've looked for the places where vec was referenced and tried to do similar things, although it turns out the implementation is closer to rust::Box. My understanding of what I'm doing is based on git grep and passing tests, and is not at all comprehensive.

• Is this on the right track? Asking another way, do you look at this and thing "what is this nonsense" or "ok, it's a start"?
• What issues, soundness or otherwise, have I not addressed?
  • One that comes to mind is ZSTs, which have non-zero pointer values that cannot be dereferenced. But, I think those are not allowed in rust::Box anyway?
• On what should I model the methods for rust::Option -- std::Option or std::optional? or neither?
• What else needs done? I know I need to write a .html page, and there are a few TODOs in the PR that I will get to.
  • I do need to add tests for Rust returning Option<Box<T>>.

[djmitche]

Well, in the absence of any negative feedback, I guess I'll put this up for review :)