The following function can push 16 bits of data to the shift registers (assuming two shifts registers cascaded)

ShiftOut

serOut=2 --SER pin
clk=1 --shift register clock (SRCLK)
latch=3 -- Storage register clock (RCLK)
loadStatus=0 --use only 16 bits

gpio.mode(clk,gpio.OUTPUT)
gpio.mode(serOut,gpio.OUTPUT)
gpio.mode(latch,gpio.OUTPUT)

local function sendClk()
    gpio.write(clk, gpio.HIGH)
    tmr.delay(5)
    gpio.write(clk, gpio.LOW)
end

local function sendLatch()
    gpio.write(latch, gpio.HIGH)
    tmr.delay(5)
    gpio.write(latch, gpio.LOW)
end

local function serialOut(data)-- will shift out 16 bits,serially
    for i=0,15 do    
     if bit.isclear(data,i) then gpio.write(serOut, gpio.LOW)
     else gpio.write(serOut, gpio.HIGH) end
     tmr.delay(5)
     sendClk()
    end
    sendLatch()
end