Promote yang.data.impl.SchemaTracker

yang-data-impl's SchemaTracker provides a common interface used by yang-data-codec-{gson,xml} when interacting with AbstractCodecFactory, which in turn interacts with SchemaContextUtil.

SchemaTracker really follows NormalizedNodeStreamWriter contract's view on NormalizedNode, hence it is fair to provide it as NormalizedNode counterpart of SchemaInferenceStack. From naming perspective, NormalizedNodeInferenceStack
 is probably a good-enough name.