Bug squashing.

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.