Have Registerable provide as `unregister()`

[schlessera]

@GaryJones suggested that an Registerable object should be able to be unregister()ed as well.

[schlessera]

I prefer to not provide an unregister() because of the following reasons:

1. In most instances, registering something to hook it up to the system is much simpler than unhooking it again after the system has been run and is in an unknown state. If the interface needs an unregister(), it forces all components to think about all the possible ways an unregister() could go wrong.
2. If the unregister() is needed to be able to turn an individual component on or off depending on context, this has more to do with the state of that component, as it has with whether the component is "known" in the system.
3. As we're dealing with PHP, it can be assumed that the entire system is always discarded after each request. So unregister()ing because of memory considerations is of little use, especially as this still does not free the memory, but marks it as available to the garbage collector.

cc @GaryJones

[GaryJones]

Then, would you consider an Unregisterable interface, so some consuming objects can both implement register and unregister?