remove pointless const_cast

Change-Id: I6026ab64f0cfe1d509c8f94ce3b9c6f700c823cf
make sax_fastparser use libxml2 instead of expat (WIP)

Not really finished but working (all unittests pass except for the one
in sw which is probably rather writerfilter bug).

Change-Id: If5c00aeaa39c50d3cdfa2965bec2d38cb494d19c