Consider relooping recursive LetDefs on JS CPS

If we remember which `LetDef`s are recursive, we can also reloop recursive functions (most often workers discovered by previous opt phases):

Source program:
```scala
def myeach[A](list: List[A]) { f: A => Unit }: Unit = list match {
case Cons(x, xs) => f(x); myeach(xs) {f}
case Nil() => ()
}

def main() = {
[(0, 'a'), (1, 'b'), (2, 'c')].myeach {
case Tuple2(i, x) => println(show(i) ++ ": " ++ show(x))
}
}
```

So it'd mean turning the `myeach_worker_...` below
```scala
def main_7486 = { | ks_10277, k_10278 =>
def myeach_worker_10204 = { (list_10205) | ks_10279, k_10280 =>
list_10205 match {
case Cons_2895 (x_10213, xs_10214) => {
x_10213 match {
case Tuple2_424 (i_10249, x_10250) => {
let! ret_10251 = showBuiltin_45(i_10249)
let! ret_10253 = showBuiltin_49(x_10250)
let! v_r_10257 = println_32(infixPlusPlus_62(infixPlusPlus_62(ret_10251, ": "), ret_10253))
myeach_worker_10204(xs_10214) @ ks_10279, k_10280
}
}
}
case Nil_2894 () => {
jump k_10280(()) @ ks_10279
}
}
}
myeach_worker_10204(make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(0, 97), make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(1, 98), make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(2, 99), make List_2564 Nil_2894())))) @ ks_10277, k_10278
}
```

into a `letrec`:

```scala
def main_7486 = { | ks_10399, k_10400 =>
letrec myeach_worker_10326 = { (list_10327) | ks_10401 =>
list_10327 match {
case Cons_2895 (x_10335, xs_10336) => {
x_10335 match {
case Tuple2_424 (i_10371, x_10372) => {
let! ret_10373 = showBuiltin_45(i_10371)
let! ret_10375 = showBuiltin_49(x_10372)
let! v_r_10379 = println_32(infixPlusPlus_62(infixPlusPlus_62(ret_10373, ": "), ret_10375))
jump myeach_worker_10326(xs_10336) @ ks_10401
}
}
}
case Nil_2894 () => {
jump k_10400(()) @ ks_10401
}
}
}
jump myeach_worker_10326(make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(0, 97), make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(1, 98), make List_2564 Cons_2895(make Tuple2_292 Tuple2_424(2, 99), make List_2564 Nil_2894())))) @ ks_10399
}
```

and then teaching the CPS IR -> JS transformer how to deal with these self loops.
Though note that in this example, it seems to have helped only by eliminating a single function call by replacing it with mutable state...

I do have a prototype of this locally, but the code is very low quality.