runtime: Add C API shim for create_partition_by_field.
runtime: Add C API shim for create_partition_by_field.

sdalton1/legion,chuckatkins/legion,StanfordLegion/legion,StanfordLegion/legion,chuckatkins/legion,chuckatkins/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,StanfordLegion/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,sdalton1/legion,StanfordLegion/legion,chuckatkins/legion,sdalton1/legion,StanfordLegion/legion,sdalton1/legion,StanfordLegion/legion