Feature request: Type by class with private memory

[MikaelMayer]

I would love to see this:

type X {
  // interface
} by class {
  // implementation by private variables
}

See the RFC here, where I prove how it's possible to make certain parts of the memory private.

https://github.com/dafny-lang/rfcs/pull/14/files