Adding some sort of validation API to CCK

Ideally actually this is a core feature, but since CCK is the only part that's dealing directly with fields atm, I've filed it here.

Basically, as I was going through and adding all CCK add-on modules to the new CCK module category, I noticed these three in the list:

- Phone (CCK)
- Social Insurance Number (CCK)
- zipcode (CCK)

All these are are essentially just text fields with special validation rules.

So it would be cool to authors to be able to develop validation routines, and we could choose from among them when creating/editing a field.