Associate partitioning information with each RelOptInfo.

 Associate partitioning information with each RelOptInfo.
 
 This is not used for anything yet, but it is necessary infrastructure
 for partition-wise join and for partition pruning without constraint
 exclusion.
 
 Ashutosh Bapat, reviewed by Amit Langote and with quite a few changes,
 mostly cosmetic, by me.  Additional review and testing of this patch
 series by Antonin Houska, Amit Khandekar, Rafia Sabih, Rajkumar
 Raghuwanshi, Thomas Munro, and Dilip Kumar.
 
 Discussion: http://postgr.es/m/CAFjFpRfneFG3H+F6BaiXemMrKF+FY-POpx3Ocy+RiH3yBmXSNw@mail.gmail.com