Remove DocumentedNode and its subclasses (SchemaNode, etc.)

DocumentedNode view of the world is an antiquated design, which is superseded by ModelStatement and its two specializations, DeclaredStatement and EffectiveStatement.

When DocumentedNode aimed to have an object model easy to use in Binding Spec, it has a number of quirks which make it hard (and inefficient) to use in other settings.

While EffectiveStatements are not as feature-rich yet, they are very close to providing everything needed for users to use them instead of SchemaNode.

Evolve both API contracts so that switching users becomes possible without wholesale removals, enriching EffectiveStatements with metadata they do not yet hold. The end goal here is removal of DocumentedNode and all interfaces which directly reference it.