Research supported by NSF Graduate and Postdoctoral Research Fellowships and NSF grants DMS-0228807, DMS-0555869, DMS-0901620, DMS-1201875, and DMS-1600650, and Simons Collaboration Grant 353627.

Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Simons Foundation.