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

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

```scala
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:

https://github.com/effekt-lang/effekt/blob/72702b30a79eb7dee7f3478ef65be68076dd2d3b/libraries/common/heap.effekt#L26

ANF-ing the `s.eq(k, 0)` like this:
```scala
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"?

Start with the minimal IntSet reproducer in the issue and compare its record-field call with the working heap.effekt example linked from #585. Trace how a boxed function is called through a record field, including the ANF variant, and consider the expected direct-call behavior or diagnostic; done when the direct form works or reports a clear remedy.

Written by the indexing model from the issue text.