Integrate AugmentationHolder into Augmentable

Since we have predictable class loading, there is no need to play weird class loader tricks: we do not need a class reference grow representations. All implementations already use AugmentationHolder, hence there is no point in pretending these contracts are separate.

Merge them and eliminate AugmentationHolder, along with any complexity it implies. This means Augmentable.augmentation() should become a default method calling down to Augmentable.augmentations().