Enable pickling for symbolic expressions

In pytential.symbolic.primitives,

• For each subclass of Expression with overrided __new__, either add default value to all positional arguments, or add __getnewargs__.
• For each subclass of Expression, add init_arg_names, so that __setstate__ in Expression can be processed correctly.
• Investigate why after pickling, IntG.kernel_arguments changes from a dictionary to a tuple.