regent: Add C API example with reductions.
regent: Add C API example with reductions.

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