SAX overhaul for HAX integration.

* dom/dom-builder.lisp (DOM-BUILDER): Inherit from
	sax:content-handler.

	* klacks/klacks-impl.lisp (KLACKS-DTD-HANDLER): Inherit from
	sax:default-handler.

	* klacks/klacks.lisp (KLACKS:SERIALIZE-EVENT): Bugfix -- call
	start-cdata and end-cdata on the handler, not the source.

	* xml/package.lisp: New export WITH-OUTPUT-SINK.

	* xml/sax-handler.lisp (SAX): New exports abstract-handler,
	content-handler, default-handler.  (STANDARD-ATTRIBUTE): Renamed
	from attribute.  (ATTRIBUTE-NAMESPACE-URI, ATTRIBUTE-LOCAL-NAME,
	ATTRIBUTE-QNAME, ATTRIBUTE-VALUE, ATTRIBUTE-SPECIFIED-P): Wrapper
	methods for standard-attribute.  Wrapper methods for
	hax:standard-attribute.  (all events): pulled into a common
	define-event form.  New dummy method on null.  Added a warning to
	the default method.   New error method on abstract-handler.  New
	dummy method on the respective default handler classes.

	* xml/sax-proxy.lisp (BROADCAST-HANDLER): Inherit from
	abstract-handler, not sax-parser-mixin.

	* xml/unparse.lisp (sink): Inherit from sax:content-handler.
	(WITH-OUTPUT-SINK): New macro.  (INVOKE-WITH-OUTPUT-SINK): New
	function.

xml/sax-proxy.lisp

@@ -8,7 +8,7 @@
 
 (in-package :cxml)
 
-(defclass broadcast-handler (sax:sax-parser-mixin)
+(defclass broadcast-handler (sax:abstract-handler)
   ((handlers :initform nil
 	     :initarg :handlers
 	     :accessor broadcast-handler-handlers)))