Use a phantom type to indicate task/scheduler context in Scheduler

[brson]

Many methods in Scheduler either assert!(self.in_task_context()) or assert!(!self.in_task_context()). This would be more elegantly expressed as a phantom type, Scheduler<TaskCtxt>, Scheduler<SchedCtxt>.

[emberian]

I do not see any such asserts or similar funcionality in rt/sched.rs, closing.