feature request: support for simple semaphore

[ncopa]

It would be handy with a simple semaphore module so it is easy to limit number of concurrent running threads, without needing to reimplement it every time.

local condition = require"cqueues.condition"

local semaphore = {}
function semaphore.new(n)
        local self = { cond = condition.new(), count = n or 1 }
        return setmetatable(self, {__index = semaphore})
end

function semaphore.aquire(self)
        while self.count == 0 do
                self.cond:wait()
        end
        self.count = self.count - 1
end

function semaphore.release(self)
        self.count = self.count + 1
        self.cond:signal()
end

[daurnimator]

I feel like this would be a good sort of thing to distribute as a separate module. There was talk about moving some of the cqueues submodules out to separate rocks.