iterx: Change field order

Signed-off-by: Michał Matczuk <michal@scylladb.com>

iterx.go

@@ -34,12 +34,13 @@ func Select(dest interface{}, q *gocql.Query) error {
 // Iterx is a wrapper around gocql.Iter which adds struct scanning capabilities.
 type Iterx struct {
 	*gocql.Iter
-	err error
+	Mapper *reflectx.Mapper
 
 	unsafe  bool
-	Mapper *reflectx.Mapper
-	// these fields cache memory use for a rows during iteration w/ structScan
 	started bool
+	err     error
+
+	// Cache memory for a rows during iteration in StructScan.
 	fields [][]int
 	values []interface{}
 }