* Don't need to sleep if queue>sleepers, can just pop the right queue
element and return it.
* If queue>sleeping and waking==sleeping, no mechanism existed to ensure
a thread newly added to sleeping would ever be woken. Above change
fixes that.
* Cannot trust select() return value, scheduler might sleep us
indefinitely while put() writes a byte.
* Sleeping threads didn't pop FIFO, they popped in whatever order
scheduler woke them up. Must recover index and use it to pick the pop
index.
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