Continue porting period_helper to cython
This moves functions to cython but retains their C names to make comparison easier.  The next pass will get rid of the camelCase.  A couple of redundant functions are left behind in period_helper, will be cleaned up in a follow-up.

The next hurdle is to get rid of all the adding and subtracting of ORD_OFFSET, WEEK_OFFSET, BDAY_OFFSET.  (The period_helper code puts 0 at Jan 1 0001 instead of Jan 1 1970).  This would be straightforward but I want to be careful about C division/mod conventions.