Ported prefix_sum to be more parallel.
Ported prefix_sum to be more parallel.

TODO: make more generalizeable way to switch between striped and blocked
distributions like this prefix_sum does.

alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,buaasun/grappa,uwsampa/grappa,uwsampa/grappa,uwsampa/grappa,uwsampa/grappa,buaasun/grappa,alexfrolov/grappa,buaasun/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,alexfrolov/grappa,alexfrolov/grappa