Simple sync/async emitter

This differs from the built-in events emitter because:

• quick and simple - only one data item passed with message
• no special-case logic for error
• can have async emits, allowing the emitter to await the processing

It's only for me. Don't use it.

Subclass from this, or create a new one

Adds a handler for the event type. Returns a function to remove.

Removes the handler for this event type.

Returns a promise which resolves the first time this event is called

Emits the event to all handlers, which are run synchronously.

Emits the event to all handlers, resolving when they have all completed.

That's it. It's simple. Don't use it.