Mark the flags `volatile`, just to be safe. This is not a problem now, but I would hate it to become one later.

switch to using a 32-bit flag field and atomics, replace '|=' with '= ( ... || ... )', which should do short-circuit evaluation, and is easier to read.

Don't pass enums in the `cell_*_flag` functions, but rather use the type of the `flags` value itself. Using the enum causes problems when passing the union of more than one flag.

replace c->stars.do_stars_sub_sort, do_drift and do_sub_drift with cell_flag_do_stars_sub_sort, do_grav_drift and do_grav_sub_drift, respectively.

replace c->hydro.do_sub_limiter with cell_flag_do_hydro_sub_limiter. also use uint16_t for the sort masks.

add an enum with the different cell flags (basically whatever was a char type and looked like a binary flag). will start replacing the cell values in the following commits.

replace the cell's mpi.limiter.recv task pointer with the new linked recv pointers. goodbye, mpi.limiter!

replace the cell's mpi.stars.recv and recv_ti task pointers with the new linked recv pointers. goodbye, mpi.stars!

replace the cell's mpi.grav.recv and recv_ti task pointers with the new linked recv pointers. added some utility functions for looking up send/recv tasks of a given subtype in a cell.

replace the cell's mpi.hydro.recv_ti task pointer with the new linked recv pointers. goodbye, mpi.hydro!

merge all send tasks of all types into a single linked list. this is easier than doing the recv tasks since they are already linked lists to begin with.

collapse the hydro send tasks into one single linked list, modify scheduler_activate_send to look for a specific task subtype.

collapse send/recv tasks into unions. this will probably break a thing or two given that we sometimes just check if these are NULL.