Added Iterator interface and two implementation: list itrator and range iterator

iterator.go

@@ -0,0 +1,37 @@
+// iterator.go
+package expr
+
+import "io"
+
+type Iterator interface {
+	Reset()
+	Next() (item any, err error) // must return io.EOF after the last item
+	Index() int
+}
+
+type FlatArrayIterator struct {
+	a     []any
+	index int
+}
+
+func NewFlatArrayIterator(array []any) *FlatArrayIterator {
+	return &FlatArrayIterator{a: array, index: 0}
+}
+
+func (it *FlatArrayIterator) Reset() {
+	it.index = 0
+}
+
+func (it *FlatArrayIterator) Next() (item any, err error) {
+	if it.index < len(it.a) {
+		item = it.a[it.index]
+		it.index++
+	} else {
+		err = io.EOF
+	}
+	return
+}
+
+func (it *FlatArrayIterator) Index() int {
+	return it.index - 1
+}