Support ? for Option and Result

[tjhance]

Internally, rustc de-sugars the 'x?' operation to something like this:

match x.branch() {
    ControlFlow::Break(b) => { return from_residual(x); }
    ControlFlow::Continue(c) => c,
}

To support '?', we just need to add support for all the content in the de-sugar:

• The std::ops::ControlFlow enum. This is easy with external_type_specification.
• The std::convert::Infallible enum. This is an empty enum, which Verus doesn't support, but we can mark it external_body.
• Option's implementation of the std::ops::Try trait (specifically the branch function).
  • Option's implementation, Source
• Option's implementation of the FromResidual trait
  • Option's implementation, Source

Result should work the same way, but the problem is that the Result's implementation of FromResidual is more generic than we support right now. It's a similar problem to vec indexing, and we can work around it the same way, but this PR doesn't handle it.

UPDATE: Result is now supported.

[tjhance]

(merged this into the wrong branch ... I'll fix it manually)

[tjhance]

I forgot that this is dependent on #949. So, I will merge this into main after #949 is in.

[tjhance]

Now merged in, for real, via 49cd0a5