Add DIY Binder example (#999)

I quickly sketched this after talking to @jiribenes about how we could
potentially add custom implementations of something like `while` with
binders. (He said I should add it as an example.):

```effekt
effect lookupX(): Int

def whil{ cond: [R]{ => R / lookupX }{ => R } => R }{ body: => Unit / lookupX }: Unit = {
  cond{ body(); whil{cond}{body} }{ () }
}

def main() = {
  var x = Some(12)
  // desugared cond
  def cond[R]{thn: => R / lookupX}{els: => R} = {
    if(x is Some(y)){ 
      try { thn() } with lookupX{ () => resume(y) }
    } else { els() }
  }
  whil{ cond }{
    println(do lookupX())
    x = Some(do lookupX() - 1)
    if (do lookupX() == 0) {
      x = None()
    }
  }
  x = Some(12)
  whil{ cond }{
    println(do lookupX())
    x = Some(do lookupX() - 1)
    if (do lookupX() == 0) {
      x = None()
    }
  }
}
```

Surprisingly (to me), this generates reasonable JS:
```javascript
function main_2(ks_8, k_7) {
  const x_2 = ks_8.arena.fresh(new Some_0(12));
  function whil_worker_6(ks_9, k_5) {
    whil_worker_7: while (true) {
      const s_3 = x_2.value;
      switch (s_3.__tag) {
        case 1: 
          const y_3 = s_3.value_0;
          const tmp_5 = $effekt.println(('' + y_3));
          x_2.set(new Some_0((y_3 - (1))));
          if (y_3 === (0)) {
            x_2.set(new None_0());
            /* prepare call */
            continue whil_worker_7;
          } else {
            /* prepare call */
            continue whil_worker_7;
          }
          break;
        default:  return () => k_5($effekt.unit, ks_9);
      }
    }
  }
  return whil_worker_6(ks_8, (_1, ks_10) => {
    function whil_worker_8(ks_11, k_6) {
      whil_worker_9: while (true) {
        const s_4 = x_2.value;
        switch (s_4.__tag) {
          case 1: 
            const y_4 = s_4.value_0;
            const tmp_6 = $effekt.println(('' + y_4));
            x_2.set(new Some_0((y_4 - (1))));
            if (y_4 === (0)) {
              x_2.set(new None_0());
              /* prepare call */
              continue whil_worker_9;
            } else {
              /* prepare call */
              continue whil_worker_9;
            }
            break;
          default:  return () => k_6($effekt.unit, ks_11);
        }
      }
    }
    return whil_worker_8(ks_10, (tmp_7, ks_12) => () => k_7(tmp_7, ks_12));
  });
}
```

Author: marzipankaiser

effekt/jvm/src/test/scala/effekt/core/VMTests.scala

@@ -819,6 +819,21 @@ class VMTests extends munit.FunSuite {
       shifts = 0,
       resumes = 0
     )),
+    examplesDir / "pos" / "diy_binder.effekt" -> Some(Summary(
+      staticDispatches = 28,
+      dynamicDispatches = 0,
+      patternMatches = 28,
+      branches = 26,
+      pushedFrames = 1,
+      poppedFrames = 1,
+      allocations = 30,
+      closures = 0,
+      variableReads = 28,
+      variableWrites = 29,
+      resets = 0,
+      shifts = 0,
+      resumes = 0
+    )),
   )
 
   val testFiles: Seq[(File, Option[Summary])] =

examples/pos/diy_binder.check

@@ -0,0 +1,26 @@
+12
+11
+10
+9
+8
+7
+6
+5
+4
+3
+2
+1
+0
+12
+11
+10
+9
+8
+7
+6
+5
+4
+3
+2
+1
+0

examples/pos/diy_binder.effekt

@@ -0,0 +1,32 @@
+effect lookupX(): Int
+
+// With parametric polymorphism on effects, this could be
+// def whil[E: EffectSet]{ cond: [R]{ => R / E }{ => R } => R }{ body: => Unit / E }: Unit
+def whil{ cond: [R]{ => R / lookupX }{ => R } => R }{ body: => Unit / lookupX }: Unit = {
+  cond{ body(); whil{cond}{body} }{ () }
+}
+
+def main() = {
+  var x = Some(12)
+  // desugared cond
+  def cond[R]{thn: => R / lookupX}{els: => R} = {
+    if(x is Some(y)){ 
+      try { thn() } with lookupX{ () => resume(y) }
+    } else { els() }
+  }
+  whil{ cond }{
+    println(do lookupX())
+    x = Some(do lookupX() - 1)
+    if (do lookupX() == 0) {
+      x = None()
+    }
+  }
+  x = Some(12)
+  whil{ cond }{
+    println(do lookupX())
+    x = Some(do lookupX() - 1)
+    if (do lookupX() == 0) {
+      x = None()
+    }
+  }
+}