[giac] use instanceof instead of isNumber/ListValue() to avoid evaluate()

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25898 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Faster serialization for x-x, x*x

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25899 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e