Correctly stringify tuple subscripts.

Python turns multiple subscripts separated by commas into a tuple of
subscripts. When stringifying this, Pymbolic inserts the set of
parentheses associated with that tuple inside the square brackets for
the subscript. This commit causes Pymbolic to suppress these
parentheses.

This fixes two issues.

- First, __getinitargs__ does not actually get called for new-style
  classes. Whoops. Only __getstate__ and __setstate__ do. This
  introduces a version of these methods that relies on a new class
  attribute init_arg_names and the existing __getinitargs__ methods
  to provide pickling.

- The default implementation of __getstate__ would pickle up the cached
  hash_value, which will likely be different in a new interpreter.
  Bad idea, fixed as above.