Now compatible with libxml2 2.12.0 and later (@KNnut).

Fixed format string issues detected in R-devel.

Remove unused dependencies on glue, withr and lifecycle (@mgirlich).

print() is faster for very long xml_nodeset inputs (#366, @MichaelChirico).

xml_attr(), xml_attrs(), xml_double(), xml_integer(), xml_length(),
xml_name(), xml_path(), xml_text(), and xml_type() no longer use
S3 dispatch but instead dispatch in C, leading to considerable performance
improvements in many cases (@mgirlich, #400).

xml_find_int() analogous to xml_find_num() for returning integers
matched by an XPath (#365, @MichaelChirico).

xml_serialize() now includes the document type so that xml_unserialize()
works also for HTML documents (#407, @HenrikBengtsson).