iterator method of treeSet has goroutine leaks

[zonewave]

I use "goleak" to detector leak:

	t.Run("diff set", func(t *testing.T) {
		defer goleak.VerifyNone(t)
		t1 := TreeSetFrom[int, Compare[int]]([]int{1, 2, 3}, Cmp[int])
		t2 := TreeSetFrom[int, Compare[int]]([]int{4, 5, 6}, Cmp[int])
		must.False(t, t1.Subset(t2))
	})

if not iter all node, the channel will block

[shoenig]

Good catch, I had been thinking about ways to iterate 2 trees (for the .Equal and .Subset cases) without the use of goroutines, but after so many years of programming in Go using goroutines is just easier.

If we could eliminate the use of goroutines though, that would be even better!