moved a subset of source file to the kern package

iter-factory.go

@@ -4,19 +4,23 @@
 // iter-factory.go
 package expr
 
-func NewIterator(value any) (it Iterator, err error) {
+import (
+	"git.portale-stac.it/go-pkg/expr/kern"
+)
+
+func NewIterator(value any) (it kern.Iterator, err error) {
 	if value == nil {
 		return NewArrayIterator([]any{}), nil
 	}
 
 	switch v := value.(type) {
-	case *ListType:
+	case *kern.ListType:
 		it = NewListIterator(v, nil)
-	case *DictType:
+	case *kern.DictType:
 		it, err = NewDictIterator(v, nil)
 	case []any:
 		it = NewArrayIterator(v)
-	case Iterator:
+	case kern.Iterator:
 		it = v
 	default:
 		it = NewArrayIterator([]any{value})