shorten code, add comment

timer.lua

@@ -32,14 +32,14 @@ local function _nothing_() end
 function Timer:update(dt)
 	local to_remove = {}
 
-	local handles = {}
+	-- timers may create new timers, which leads to undefined behavior
-	local len = 0
+	-- in pairs() - so we need to put them in a different table first
+	local to_update = {}
 	for handle in pairs(self.functions) do
-		len = len + 1
+		to_update[handle] = handle
-		handles[len] = handle
 	end
 
-	for _, handle in ipairs(handles) do
+	for handle in pairs(to_update) do
 		-- handle: {
 		--   time = <number>,
 		--   after = <function>,