keep_blank_values option to parse().

The optimizations consist mostly of using local variables to cache
methods or instance variables used a lot (e.g. "self.write").

The loopholes allows marshalling extension types as long as they have
a __class__ attribute (in which case they may support the rest of the
class piclking protocol as well).  This allows pickling MESS extension
types.

low on stackspace.

creation; no longer support this for the config method.