Context argument for stream write operation

[moteus]

This may be helpful to implement wrapper classes

E.g. SSL socket

function SSLSocket:write(msg,cb)
  -- here we have to make closure for each call 
  -- (or use table to cache them with `cb` as key)
  self._sock:write(msg, function(_, ...) cb(self, ...) end)
end

I suggest allow pass additional context argument to write function

function SSLSocket:__init()
  self._on_write_done = function(_, err, cb) cb(self, err) end
end

function SSLSocket:write(msg,cb)
  -- here we can use same closure for all calls
  self._sock:write(msg, self._on_write_done, cb)
end

Add context to other callbacks I see no reason because they do not call as often as write.
But this adds some inconsistent to API. Also I do not think it possible make multiple variables as context because for this we need some table and user code can do this by self.