Add the PureXML.Catalog namespace: parse an OASIS XML catalog (public,
system, uri, rewriteSystem, rewriteURI, group/catalog by local name) into a
Resolver mapping public/system identifiers and URI names to URIs, with
longest-prefix rewriting and system-over-public precedence. entityResolver
(loadingURI:) builds a Parsing.EntityResolver that resolves an external
identifier through the catalog and loads the URI via an injected closure, so
a catalog plus loader wires entities and DTDs in while the default keeps XXE
closed.
Closes#57Closes#58Closes#27