`If` behaviour on stuck terms

[o-santi]

Currently, the (If ...) rule on the genesis block evaluates everything that is strictly not #0 to the true branch. This includes terms that cannot reduce any further, which can cause some very unexpected behaviour when comparing objects of different types.

For example, the following code:

fun (Test x) {
  (Test x) = (If (== x #5) (Done 'yay') (Fail #0))
}

run {
  ask test = (Call 'Test' [#4])
  (Done test)
}

will silently fail. It will always evaluate to (Done 'yay') even though it clearly is not the intended semantics, because the following comparison (== {T1 #4} #5) gets stuck, and If evaluates that to true.

I do believe that we should change it match only on #0 and #1, in order to avoid javascript-esque errors like these. But I think that this raises a bigger question: what should be the norm/expected behaviour on treating stuck terms?

cc @steinerkelvin @developedby