Department of Computer Science, Harvey Mudd College, Claremont, CA 91711, Department of Electrical Engineering and Computer Science, MIT, Cambridge, MA 02139, Department of Computer Science and Engineering, University of Connecticut, Storrs, CT 06269 and Broad Institute, Cambridge, MA 02142, USADepartment of Computer Science, Harvey Mudd College, Claremont, CA 91711, Department of Electrical Engineering and Computer Science, MIT, Cambridge, MA 02139, Department of Computer Science and Engineering, University of Connecticut, Storrs, CT 06269 and Broad Institute, Cambridge, MA 02142, USA.