A document() reference with a #fragment selects a subset of the loaded
document via XPointer; a node-set first argument unions the documents
named by each node's string value; and a relative URI is resolved against
a caller-supplied baseURI (the loader is wrapped so xsl:include/xsl:import
resolve the same way). Closes the XSLT function/output audit.
The text content of the named result elements is emitted in CDATA
sections rather than escaped: the result tree's text children of those
elements are converted to CDATA nodes before serialization, leaving
nested and other elements unaffected. Part of #82.
When doctype-system (optionally with doctype-public) is set on xsl:output,
a <!DOCTYPE> for the result's root element is emitted before the
serialized output: PUBLIC when both identifiers are given, SYSTEM
otherwise. Works for the XML and HTML output methods. Part of #82.
An xsl:output method=html result is serialized through PureXML.HTML
(void elements emitted without a self-closing slash, raw-text elements
like script/style left unescaped) instead of as XML. Part of #82.
xsl:message instantiates its body as diagnostic text and produces no
result-tree output; terminate=yes records the text and aborts the
transform with an XSLTError.terminated, halting further instantiation.
Splits the transform entry points into XSLTTransform.swift and moves the
message case to keep within the complexity and file-length budgets.
