Demo control-flow operator

[jsiek]

As we refactor the interpreter, we need to make sure that we don't accidentally box ourselves in with
and approach that can't handle the control-flow features that we may want to add in the future, such as
exceptions, question mark, coroutines, threads, etc.

So the idea is to demo a fairly powerful control-flow feature now as a place-holder, such as first-class continuations (or perhaps delimited continuations), even though we may not (probably do not?) want this feature in Carbon.