Fix schedule_unique and unschedule

[ginkulv]

Right now these two functions are straight up broken. Code to reproduce:

using GameZero 
using Colors 

function draw() end 
function update() end 

function test() end 

schedule_unique(test, 1)
# or
unschedule(test)

[aviks]

Thanks. Yes, this bit is not very well tested. I'll add an example to exercise this.