timer: sleep(duration) via min-heap of (deadline, pid)

Adds a BinaryHeap of timer entries on SchedulerState. sleep() inserts
an entry and parks; schedule_loop pops due entries each iteration and
unparks them. When the run queue is empty but timers are pending, the
OS thread sleeps until the soonest deadline.

Single-threaded only; thread::sleep is fine because no other thread
can wake us. The IO thread coming next will need a Condvar or pipe
wakeup to break this OS-sleep early.

src/lib.rs

@@ -18,6 +18,7 @@ pub mod actor;
 pub mod channel;
 pub mod scheduler;
 pub mod supervisor;
+pub mod timer;
 
 // ---------------------------------------------------------------------------
 // Global allocator
@@ -36,5 +37,5 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 
 pub use channel::{channel, Receiver, RecvError, Sender};
 pub use pid::Pid;
-pub use scheduler::{run, self_pid, spawn, spawn_under, yield_now, JoinError, JoinHandle};
+pub use scheduler::{run, self_pid, sleep, spawn, spawn_under, yield_now, JoinError, JoinHandle};
 pub use supervisor::Signal;