sql comments support

@tenderlove as discussed [here](https://groups.google.com/d/msg/rubyonrails-core/-/ynWCfU9FuVgJ)

```
scope = User.where(aaa).comment("XXX")
scope.comment("YYY")
scope.to_sql == "SELECT * FROM users WHERE xxx /* XXX YYY */"
```
maybe this should be implemented as a separate comment node?