use libxml2 memory management throughout the file, which in turn uses the
PHP memory management.

send some more data to the default handler
libxml2 is braindead when it comes to handling entities

send comments to the default handler like done with expat, add a
setcommenthandler function, which exists in expat, but isn't used by the
extension.

Allow libxml to detect the document encoding if encoding parameter passed to
XML_ParserCreate_MM is null.

Filled error_mappings[] with placeholders for actual error messages

Added missing object initialisation
Added missing sanity checks

Fixed typos

This should be needed.

Added even better allocator codes for both expat & libxml

Bundle libxml and add compatibility layer