* Support debug-logging to stderr by defining the MATRIX_EVENTABLE_DEBUG_LOG
environment variable.
* Support for unhooking handlers from events.
* Simplify the low-level interface: eventable.functions() now returns the
created functions directly, cutting down on an intermediate table.
* A new eventable.object() allows adding :hook(), :unhook() and :fire()
methods to any table. Firing events always sends the table itself as first
event parameter when firing any event.
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