Warn on usage of ChildSingleton inside of singletons

[tessig]

If a binding is declared as dingo.ChildSingleton, it has different instances for each area. If a dingo.Singleton gets a dingo.ChildSingleton via injection, it is unclear from which area the instance will come and can lead to unwanted behaviour.

We should declare this as misusage of the ChildSingleton pattern and at least warn the user.