Get rid of date_part_is_valid() which got too recursive and stick with just...

Get rid of date_part_is_valid() which got too recursive and stick with just date_is_valid(); move hook_elements() to date_forms.inc so all element code is together for clarity and easier debugging; break up processing in Date module so each process is for a single element for consistency with the way things will work in the 6.x CCK port; more cleanup of the date processing and validation;