Solve some nasty deschedulinging races with a lock

Right now, as pointed out in #8132, it is very easy to introduce a subtle race
in the runtime. I believe that this is the cause of the current flakiness on the
bots.

I have taken the last idea mentioned in that issue which is to use a lock around
descheduling and context switching in order to solve this race.

Closes #8132
Rewrite std::comm

* TODO: performance numbers
* 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