XMLisp: don't compute the filename from scratch for each XML node.
XMLisp: cache results of make-xmlisp-symbol to reduce string manipulation.
XMLisp: optimize formatting of attribute & tag names and string escaping.
Cache printable slot lists for abstract-item subclasses.
XMLisp: cache reverse resolution of classes to tag symbols.
XMLisp: do not search for a slot definition when likely already having one.