This adds a timer(<seconds>) function, which causes an event of type
"timer" to be fired after that many seconds has elapsed.
Because it's node timer based, it works properly across server restarts
and block unloading. Thus, simplest example, a blinky plant replacement
with a 10 second period:
if event.type == "program" then
timer(10)
elseif event.type == "timer" then
port.a = not port.a
timer(10)
end
Example of problem fixed by this: Edit lua code, press Execute. Now
(execute button has focus), hold down a key. Zillions of "program"
events are generated.