Moving the `promises` and `expects` accessors to Context

The concept of `self.order` does not make much sense. Where did that variable or attribute reader come from? It makes a ton more sense to ask the Context itself to provide the accessors for the promised and expected keys.

I had to write this before:
```ruby
class AddsOneAction
  include LightService::Action
  expects :number
  promises :number

  executed do |context|
    number = self.number
    number += 1

    self.number = number
  end
end
```

And now it's only this:
```ruby
class AddsOneAction
  include LightService::Action
  expects :number
  promises :number

  executed do |context|
    context.number += 1
  end
end
```

It feels so much better to ask the context for values and not rely on some kind of method hanging on `self` not knowing where it came from.