Timer: After caching timers "to_update", timer:cancel() doesn't apply until next timer:update()

[oniietzschan]

Hey, you may recall this PR: #94

Turns out that it actually introduced a new issue which can be demonstrated with the following code:

local timer = require 'lib.hump.timer'()

local handles = {}
local function newTimer(index)
  local newHandle = timer:after(1, function()
    print('-> excuting timer #' .. index)
    -- Whichever timer executes first will cancel all of the other timers.
    for _, handle in ipairs(handles) do
      timer:cancel(handle)
    end
  end)
  table.insert(handles, newHandle)
end

for i = 1, 10 do
  newTimer(i)
end

print('Updating timers...')
timer:update(1)
print('Finished.')

Expected Result (Which one callback actually executes is contingent, of course.)

Updating timers...
-> excuting timer #9
Finished.

Actual Result (Again, the order here is contingent)

Updating timers...
-> excuting timer #6
-> excuting timer #5
-> excuting timer #2
-> excuting timer #8
-> excuting timer #10
-> excuting timer #1
-> excuting timer #4
-> excuting timer #7
-> excuting timer #3
-> excuting timer #9
Finished.

This issue can be fixed by adding a check like if self.functions[handle] == nil then <continue> end during the primary loop in Timer:update().

I didn't send you a PR because there are a few different styles of achieving a "continue" in Lua, and I wasn't sure which you'd prefer.

[vrld]

Thanks!