Updated patch:

 - make hash(m/P) preserve sign, as discussed earlier
 - add details for computing the hash of a complex number
 - reorganize sys.hash_info
    - drop sys.hash_info.bits (the exponent of the Mersenne prime);
      it's not needed in the Python code, and it can be deduced from
      the prime itself if necessary.  This also means that there's no
      public requirement that the prime be a Mersenne prime.
    - drop sys.hash_info.ninf;  just use -sys.hash_info.inf instead

    - add sys.hash_info.width:  the underlying width in bits for hashes
      of all descriptions;  in other words, it's just the number of bits
      in a C long in the current implementation
    - add sys.hash_info.imag, the multiplier used for the imaginary
      part of a complex number