Change the way of determining the MRO algorithm used from an enum

Change the way of determining the MRO algorithm used from an enum
defined in the header, to a pointer to a structure. This allows the
flexibility to easily add more MROs in the future, and to provide an
API to do so. Dispatch in mro.c is now via the structure pointed to,
rather than switch statements on the value of the enum.

p4raw-id: //depot/perl@31977