A SchemaInferenceStack.enterDataTree()

So far SchemaInferenceStack only dealt with simple steps like grouping/schema tree. In order to make it more useful in data-centric world, it also needs to support going through data tree.

While the enter/exit logic is very much similar to what schema tree does, the result is not exactly the same: toSchemaNodeIdentifier() needs to essentially double-check each statement to see if it needs actually multiple steps.