Cannot automatically unbox a boxed function if it's a record field

[jiribenes]

This is a minified version of a boxing[?]-related issue arising in #561:

record IntSet(eq: (Int, Int) => Bool at {})

def get(s: IntSet, k: Int) = {
  s.eq(k, 0) match {
//~^~~~~~~~~
// ^ Wrong number of value arguments, given 3, but eq expects 1.

    case true => <>
    case false => <>
  }
}

The actual failing example

record Map[K, V](m: MapInternal[K, V], cmp: (K, K) => Ordering at {})

type MapInternal[K, V] {
  Bin(size: Int, k: K, v: V, left: MapInternal[K, V], right: MapInternal[K, V]);
  Tip()
}

def get[K, V](map: Map[K, V], k: K): Option[V] = {
  map.m match {
    case Tip() => None()
    case Bin(size, k2, v, l, r) =>
      map.cmp(k, k2) match {
//    ~~~^~~~~~~~~~~
        case Less() => l.get(k)
        case Greater() => r.get(k)
        case Equal() => Some(v)
      }
  }  
}

---

Interestingly enough, something similar seems to work perfectly fine in the heap example in #585:

effekt/libraries/common/heap.effekt

Line 26 in 72702b3

if (idx > 0 and (heap.cmp)(arr.unsafeGet(parent(idx)), arr.unsafeGet(idx)) is Greater()) {

ANF-ing the s.eq(k, 0) like this:

val zero? = s.eq(k, 0)
zero? match { ... }

doesn't work either, not even if I annotate the new binding with : Bool.

---

Of course, one can:

1. explicitly unbox: (unbox s.eq)(k, 0), but that's not exactly pretty
2. unpack the s: val Set(eq) = s; eq(k, 0) match { ... }, but I don't like it either... :/
3. wrap in parens: (s.eq)(k, 0), thanks to @marzipankaiser for the mention :)

However, I'd really like to write it directly: is there a way to see through this layer of indirection?
Or could we at least have a nicer error message here saying "try unpacking by hand"?