Add removeListener for event emitter

[cderv]

All the function for an event emitter object has not yet be implemented. Especially, removeListener.
See spec: https://nodejs.org/api/events.html#events_emitter_removelistener_eventname_listener

it is included in once() because the listener is removed at first occurrence. But when register with on(), there is no way yet to remove the listener.

Easy way :

Consider one listener per event only. Removing the listener is equivalent to removing the event.

Hard way:

Consider several listeners per event. removeListener should be able to remove the correct listener.

Ideas:

• Currently callbacks objects return a rm function at registration.
• This function could be kept somewhere (in a list or env) and called when needed to remove the listener.
• This would require a key so that the correct listener is removed. 🤔 use digest::digest() ?

examples

myEmitter <- EventEmitter$new()
myEmitter$on('event',
    function() {
        message("an event occured")
    }
)

how to remove this listener with anonymous function ?

• myEmitter$removeListener('event", function() { message("an event occured") }) ?

Only allow it with named function?

myEmitter <- EventEmitter$new()
my_fun <-  function() {
        message("an event occured")
    }
myEmitter$on('event', my_fun)

• myEmitter$removeListener('event', "myfun") ?
• myEmitter$removeListener('event', myfun) ?