Add outer class for fix inner singleton class

That code raise NameError.

```ruby
module Mod
  module InnerModule
  end
  class InnerClass
  end
  class << self
    InnerModule #=> uninitialized constant InnerModule (NameError)
    InnerClass #=> uninitialized constant InnerClass (NameError)
  end
end
```

This reason was that singleton class object can't find outer class for iv `__outer__` was nothing.