AugmentEffectiveStatement should be SchemaTreeAwareEffectiveStatement

Downstream experience with designing support classes for various transformations, most notably with MD-SAL's binding generator, shows a gap in AugmentEffectiveStatement.

It is the only EffectiveStatement which can hold SchemaTreeEffectiveStatements, but does not expose that fact as a trait.

Update the design to be keyed off of SchemaTreeAwareEffectiveStatement, so that this trait can be used downstream – for example for generic argument bounds.