ck_hp_fifo: No need for volatile atomic stores on initialization.
ck_fifo_spsc: De-initialize from head.
ck_hp_fifo: Add deinit routine to allow reclaiming stub node.
regressions: Add basic coverage of ck_hp_fifo.
regressions/ck_hp_fifo: Require non-NULL stub node.
regressions/ck_fifo: Require non-NULL stub node.