Mark Task::resume_all as unsafe

[michael-p]

Noticed this while working on #85. Suspending/resuming the scheduler is used for many snychonization tasks within FreeRTOS (e.g. queues), so wrongly calling Task::resume_all without a matching previous call to Task::suspend_all will wreak all sorts of havoc.
Note that I did not mark Task::suspend_all as unsafe - it should be safe to call this as often as one wants, the worst thing that happens is that the scheduler just stops doing anything, but no risk for memory safety.