SessionFail should be hidden by default

[SimonJF]

The SessionFail effect makes life difficult, even if it is morally correct. This patch hides it. Fixes #1201 .

[dhil]

typename Review = [|LGTM|Reject|];
typename PullRequest = ?Review.End;

sig simon : (PullRequest) ~> Bool 
fun simon(ch) { 
  var (status, ch) = receive(ch); 
  close(ch); 
  switch (status) { 
    case LGTM -> true 
    case Reject -> false 
   }
}

sig daniel : (~PullRequest) ~> () 
fun daniel(ch) { 
  var ch = send(LGTM, ch); 
  close(ch) 
}

fun review() { 
  var ch = fork(daniel); 
  simon(ch) 
}

review()

[SimonJF]

Ha, love it!!!

[SimonJF]

CI is passing, this is approved, I will therefore take it upon myself to merge this utterly mammoth change