Add the PureXML.XInclude namespace: RFC 3986 reference resolution
(URIReference with dot-segment removal) and an xi:include processor over the
value tree. href resolves against the base and in-scope xml:base; parse=xml
includes the document element or an xpointer() fragment (via PureXML.XPointer),
parse=text includes raw text, and a failed load uses xi:fallback or errors.
Includes nest. No I/O is done internally: fetching is only through the
injected loadingURI closure, so the default fetches nothing.
Closes#48Closes#49Closes#50Closes#51Closes#24
