HTML::TreeBuilder and XPath compatible interface with libxml

 HTML::TreeBuilder::XPath is libxml based compatible interface to
 HTML::TreeBuilder, which could be slow for a large document.
 .
 HTML::TreeBuilder::LibXML is drop-in-replacement for
 HTML::TreeBuilder::XPath.
 .
 This module doesn't implement all of HTML::TreeBuilder and
 HTML::Element APIs, but enough methods are defined so modules like
 Web::Scraper work.