Request: utilities for working with dynamic frames

[robin-aws]

At the very least the Valid()/Repr idiom is not something Dafny users tend to stumble on by themselves. That idiom is also very general, and a more structured specialization of it would be good enough for a lot of Dafny code and much easier to work with.

#22 is a very basic first step towards this but unfortunately blocked by dafny-lang/dafny#1588. It may be possible to avoid that and @seebees and @jtristan have been working on an alternative.