Make ParentNodeMixin conform to the new mixin API.
Make ParentNodeMixin conform to the new mixin API.

substance/substance,TypesetIO/substance,andene/substance,michael/substance-1,substance/substance,stencila/substance,podviaznikov/substance,TypesetIO/substance,jacwright/substance,michael/substance-1,abhaga/substance,andene/substance,abhaga/substance,jacwright/substance,podviaznikov/substance