Implement some visitors

For using it in activerecord-endoscope.
https://github.com/tkawa/activerecord-endoscope

* visit_Arel_Nodes_Grouping (doubtful)
* visit_Arel_Nodes_Between
* visit_Arel_Nodes_Not (source is invalid)
* visit_Arel_Nodes_In
* visit_Arel_Nodes_NotIn (source is invalid)
* visit_Arel_Nodes_NotEqual