initialise argument value array with default values to avoid having to check for NULL on assignments

reverted latest changes: calling PyDict_GetItem inside a switch is actually faster than always looping over the kwdict

always parse keyword arguments in a PyDict_Next() loop - almost always faster than multiple calls to PyDict_GetItem()

be more optimistic when parsing optional keywords: if (interned) string pointers are identical, we know it's the right type of object

make GCC happy when string literals and const char* pointers are passed to many C-API calls or set in struct slots for Python 2.3/2.4