This is a very real problem with cvars on normal systems, and all of channels
will not work if spurious wakeups are accepted. This problem is just solved with
a synchronized flag (accessed in the cvar's lock) to see whether a signal()
actually happened or whether it's spurious.
This will prevent a deadlock when a task spins in a try_recv when using channel
communication routines is a clear location for a M:N scheduling to happen.
Like the librustuv refactoring, this refactors std::comm to sever all ties with
the scheduler. This means that the entire `comm::imp` module can be deleted in
favor of implementations outside of libstd.
* Streams are now ~3x faster than before (fewer allocations and more optimized)
* Based on a single-producer single-consumer lock-free queue that doesn't
always have to allocate on every send.
* Blocking via mutexes/cond vars outside the runtime
* Streams work in/out of the runtime seamlessly
* Select now works in/out of the runtime seamlessly
* Streams will now fail!() on send() if the other end has hung up
* try_send() will not fail
* PortOne/ChanOne removed
* SharedPort removed
* MegaPipe removed
* Generic select removed (only one kind of port now)
* API redesign
* try_recv == never block
* recv_opt == block, don't fail
* iter() == Iterator<T> for Port<T>
* removed peek
* Type::new
* Removed rt::comm
