Replace module-level factory functions with proper constructors
Sphinx can now use method docstrings
Reinstate Curl* class documentation
Pacify C compilers
Prohibit arguments to Curl* constructors, per the old API
tp_alloc is supposed to return zeroed memory, rely on this fact.

This commit adds assertions that the memory is in fact zeroed.
In the future the assertions should only be checked in debug mode.
util_curl_new is now unnecessary