Bug squashing.
* Fix setting functions to check for null value (interpreted as property
deletion) and raise an exception or do something else useful.
* Fix conversion of read buffer to string so it returns the remainder
rather than the whole thing.
* Make conversions between MP and Python long work on PyObjects rather
than PyLongObjects, to eliminate pointless type casts.
* Fix primefield constructor argument parsing so that it doesn't crash!
* Fix radix checking in MP conversions to exclude +/-1, and 0 for output.
This turns into a little function now.
* Do argument checking on Jacobi function.
~mdw / pyke / commit