parser: Rework parsing of attribute and entity values

Don't use a separate function to handle "complex" attributes. Validate
UTF-8 byte sequences without decoding. This should improve performance
considerably when parsing multi-byte UTF-8 sequences.

Use a string buffer to avoid unnecessary allocations and copying when
expanding entities.

Normalize attribute values in a single pass while expanding entities.

Be more lenient in recovery mode.

If no entity substitution was requested, validate entities without
expanding. Fixes #596.

Also fixes #655.

include/private/parser.h

@@ -87,4 +87,8 @@ XML_HIDDEN xmlParserInputPtr
 xmlNewInputPush(xmlParserCtxtPtr ctxt, const char *url,
                 const char *chunk, int size, const char *encoding);
 
+XML_HIDDEN xmlChar *
+xmlExpandEntitiesInAttValue(xmlParserCtxtPtr ctxt, const xmlChar *str,
+                            int normalize);
+
 #endif /* XML_PARSER_H_PRIVATE__ */