We still need the Helpers constant in the ActiveRecord namespace

We should probably review the usage of these constants and decide
whether we want to be more transparent about which things come from
Active Model vs Active Record, as this is meaningful information as to
whether there is database specific behavior or not.

activerecord/lib/active_record/type.rb

@@ -44,6 +44,7 @@ module ActiveRecord
       end
     end
 
+    Helpers = ActiveModel::Type::Helpers
     BigInteger = ActiveModel::Type::BigInteger
     Binary = ActiveModel::Type::Binary
     Boolean = ActiveModel::Type::Boolean