catacomb/__init__.py: Prepare rational classes for upcoming changes.

In the near future, we won't be able to guarantee that all rings allow
implicit conversions of (integer) 0 and 1 to the additive and
multiplicative ring identities, so:

  * define `ZERO' and `ONE' class attributes on the concrete classes to
    hold the respective ring identities;

  * make the `_split_rat' function into a private method of the
    `BaseRat' class and adjust callers to match;

  * compare against `ZERO' to detect an exact element of the base ring;
    and

  * return `ONE' as the implicit denominator for a base-ring element or
    foreign object to be converted.

No functional change.