bfs: port plain bfs.cpp (but GCE problems cause it to hang)
bfs: port plain bfs.cpp (but GCE problems cause it to hang)

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