immutablitiy, magic, and better typing

• support immutable event objects
• require event dispatcher to use reified generics.
• require listener providers to use reified generics.
• remove $event argument of IListenerProvider::getListeners() ( providers must use the T type instead of the object to determine listeners ) add support for immutable event objects.
• support only hhvm ~4.83.1 || ~4.84.0
• require nuxed/contract:^2.0