Analyze in which event a callback will execute

[happibum]

In an event-driven language like JavaScript, we can distinguish callback functions into
0) simple continuations that are executed by callees,

1. handlers that we only register in the current event-handling and could be executed in the next event-handling
2. handlers that are registered by possibly next event-handlers
   and so on.

Such information can be useful for

1. understanding control-flow of the program
2. identifying unnecessary serialization of events
3. finding bugs related to the event-order

The analysis result should assign a number which represent the event timing to each callback function.
We could also analyze the order dependency between callbacks.