add graph_compute_visit function and make visit structure public

src/include/66/graph.h

src/include/66/utils.h

src/lib66/exec/ssexec_disable.c

src/lib66/exec/ssexec_enable.c

src/lib66/exec/ssexec_reconfigure.c

src/lib66/exec/ssexec_start.c

src/lib66/exec/ssexec_stop.c

src/lib66/exec/ssexec_tree_admin.c

src/lib66/graph/deps-lib/deps

src/lib66/graph/graph_compute_visit.c

src/lib66/utils/deps-lib/deps

src/lib66/utils/visit.c