Improve mapping of YANG extensions

Our current handling of statements within a YANG extension performs encapsulation into UnknownNodes – this is done in the middle of SourceSpecificContext.createDeclaredChild().

As the associated FIXME indicates, the way this is done is inefficient, as we end up creating a new ModelDefinedStatementDefinition and a corresponding UnknownStatementImpl.Definition for each such instance.

In order to fix this we have to change the way we look up StatementDefinitionContext when creating a ContextBuilder. Instead of going directly to BuildGlobalContext, we should consult current StatementContextBase.definition(), which should return a StatementDefinitionContext for the requested child:

public StatementDefinitionContext getChildStatementDefinition(QName, SchemaSourceReference) {
  final StatementSupport<?, ?, ?> childSupport = support.getChildSupport(qname);
  SourceException.throwIfNull(childSupport, sourceReference, "Unsupported statement %s", qname);
... (consult & populate cache) ...
 }

There really need to be two implementations:

• one on top of BuildGlobalContext, which will contain intertwined globally-defined statements, reusing BuildGlobalContext.definitions as a cache,
• one for use with extensions, where the cache is bound to an instance of ModelDefinedStatementSupport.

This may mean we need to update what StatementDefinitionContext.substatementBuilder() does...