Missing Apalache support for Quint-definable operators

[bugarela]

Some operators like getOnlyElement and foldr are not supported by Apalache. However, they can be easily defined in Quint itself. The reason they are built-ins is to provide better performance and error messages.

q::debug also is not supported. We could add support it on the Apalache side, but it just seems more scalable in terms of maintainability to add a def like this on Quint:

pure def q::debug(s, a) = a

I hacked a first version with these:

  const builtInDefs = [
    `pure def reverse(l: List[a]): List[a] = {
    pure val len = l.length()
    l.indices().fold(l, (acc, i) => acc.replaceAt(i, l[len - i - 1]))
  }`,

    `pure def foldr(l: List[a], init: b, op: (a, b) => b): b = {
    l.reverse().foldl(init, (acc, e) => op(e, acc))
  }`,

    `pure def getOnlyElement(s: Set[a]): a = {
     pure val hack = s.map(e => ("value", e)).setToMap()
     if (s.size() != 1) {
       hack.get("error: expected singleton")
     } else {
       hack.get("value")
     }
   }`,

    `pure def allListsUpTo(s, l) = {
    1.to(l).fold(Set([]), (acc, i) => {
      pure val kms = 0.to(i - 1).setOfMaps(s)
      pure val lists = kms.map(km => range(0, i).foldl([], (acc, i) => acc.append(km.get(i))))
      acc.union(lists)
    })
  }`,

    `pure def q::debug(s, a) = a`,
  ]

But we should probably have this in a standard library (see #1314).

We also want unwrap() included like this (but this one was not even added to the spells yet)