Improve InferredStatementContext on-demand instantiation

When we are faced for a query for a particular child, we really do not want to instantiate all children.

Given we can use logic from YANGTOOLS-1157, we can now precisely know which statement from the prototype we need when asked for a SchemaTreeEffectiveStatement's StmtContext.

Initialize only that child and remember that action in a Map. When it comes time to materialize children be mindful of any such instantiations which might have happened and skip those statements.