added extra argument to EvaluationContext.keys()
added support for explicit *varargs and **varkw in the function to generate argspecs manually (argspec())
a varargs variable provided ony for having kwonly arguments is now set to None in the argspec

 In python2, we need to give the information explicitly in a class variable: kwonlyandvarkw (defaults to False)
converted dump to FunctionExpression
funcname is completed in FillArgSpec
s/func_name/funcname
more fixes to *varargs / **varkwargs handling