Use module instead of abstract base class for instantiated containers

[HertzDevil]

Turns BindgenHelper::SequentialContainer into a module. This serves two purposes:

• It prevents Crystal from reducing unions of possibly unrelated containers into SequentialContainer.
• It frees up the Crystal base class so that actual inheritance of container wrappers can be represented, should it be done in the future (e.g. Container_QQueue_int < Container_QList_int).

[Papierkorb]

Thanks!