Optional ReadonlyschemaIf given, the parsed document is run through materialize (issue #7,
src/deserialize.ts) against this schema: leaf values are upgraded
(e.g. an ISO date string to a Date) and the shape is checked
(cardinality, closedness), guaranteeing the result conforms -- or
raising ParseError with the full structured issue list otherwise.
Options for parsing OML source text into a Document node (spec §4).