callSoon() is not working prior getGlobalDispatcher().

[cheatfate]

Following code

import asyncdispatch
proc testCallback() =
  echo "testCallback()"
when isMainModule:
  callSoon(testCallback)
  poll()

must work, but generates exception:

testCallback()
asyncdispatch.nim(1507)  asyncbug
asyncdispatch.nim(1511)  poll
asyncdispatch.nim(298)   runOnce
Error: unhandled exception: No handles or timers registered in dispatcher. [ValueError]

While following code

import asyncdispatch

proc testCallback() =
  echo "testCallback()"

when isMainModule:
  var p = getGlobalDispatcher()
  callSoon(testCallback)
  poll()

working as expected, without any error.

This is regression bug introduced by one of the recent PRs, because original callSoon proc is now not exported.