runtime: insert an instanceof check to fix uncompiled tests.
datum+parse: remove apparently unneeded r5js.Datum#closestAncestorSibling.
parse: remove r5js.DatumStream#maybeRecoverAfterDeeplyNestedList and impl.
sibling_buffer+macro: remove null check from SiblingBuffer#appendSibling.
scan+read+test: remove Token class. Scanner should create datums directly.