union operation in find method, similar to the IN Operator in SQL 

Hi

I always wanted s.th. similar like the SQL Operatorfor 'IN' for ohm.

I've implemented it now for Model.find and I was wondering if you have any thoughts about it?

In our case  we have 4 slugs and instead of doing this:
```ruby
[slug1, slug2, slug3, slug4].each do
  object = Model.find(slug: slug, user_id: some_id)
  ...
end
```
We wanna do 

```ruby
Model.find(slug: [slug1, slug2, slug3, slug4], user_id: some_id).each do |object|
  ...
end
```

Could there be some performance issues? What would be more performant?
I didn't run any test yet nor didn't write any. I first would like to get some other views about this subject.

Thanks