Fine grained rspec integration

This creates an intermediary array and then throws it away. It's more code, but what about:

```ruby
@world.example_groups.each_object([]) do |example_groups, all_examples|
  example_group.descendants.each do |descendant|
    all_examples.concat(descendant.examples)
  end
end
```

Given that `descendant.examples` already exists in memory, this shouldn't result in any extra allocations besides the `all_examples` Array.