Named attribute sets apply to a literal result element or xsl:element via
use-attribute-sets; sets may include other sets recursively with a cycle
guard, set attributes are lowest precedence so the element's own
attributes override them, and same-named attributes collapse to the last.
Folded across xsl:include/xsl:import. Splits the element-building helpers
into XSLTElements.swift to stay within the file-length budget.
Add the PureXML.XSLT namespace with a stylesheet model (template rules with
match patterns and computed default priorities, the instruction sequence
constructor, attribute value templates) and a parser compiling an
xsl:stylesheet document into it. Recognizes the XSLT vocabulary by namespace
or xsl prefix; other elements become literal result elements. Covers
apply-templates, value-of, for-each, if, choose, element, attribute, copy,
copy-of, call-template, variable, text, and sort.
Closes#14
