EPP should not be defined for non-participants

[ShapeOfMatter]

Consider the choreography

asdf :: Choreo '["a", "b"] (CLI m) Int
asdf = do x <- a `_locally` getInput "Enter a number"
                 x' <- (a `introAnd` a, x) ~> a @@ b @@ nobody
                 naked refl x'

I think I've written that so it's totally valid, but what happens if we try to project it to "c"? x' will be Empty, so naked can't work.

Since we need to determine at Haskell-Runtime which party we're projecting to, I don't think we can use our existing GDP approach on epp; i don't yet know how to fix this.

[ShapeOfMatter]

My current idea is to re-write EPP to take a Member p census arg instead of a LocTm. For this to happen elegantly, we should first refactor Located so it can have an instance of TestEquality for Member. (although... having such an instance will require deciding how to handle a census that contains the same party twice.)

Then we can recover the old behavior with some kind of forLocs wrapper maybe?