grtos: check in a little bit of testing infastructure
grtos: kernel scheduler_pick_next_task call now has a type sig Ivory can implement
grtos: SysTickHandler now calls scheduler_tick, a void-void

delegate any pendsv setting to inside that code, because
event delivery already does that
grtos: priority group no longer exposes array size in type

- nice and wrapped up into module, loop, pending, send
grtos: start of System module

sketch out the types that will satisify the scheduler

probably better named Scheduler, honestly
grtos: test fixups, gitignore