Allow uniquely identifying subscriptions

[waiting-for-dev]

We add an id: param to all the methods to create subscriptions. We can
use it to fetch a given subscription from the bus when a reference to it
is no longer available. Example:

subscription = bus.subscribe(:foo, id: :foo_subscription) { do_something }
bus.subscription(:foo) == subscription # => true

That's very useful as it allows us to make the event bus the only global
piece in the code and use it for testing purposes like in the
Omnes::Bus#performing_only method.

As it's possible to subscribe multiple instances of a subscriber class
to the same bus, we also add the possibility of making IDs dependent on
the given instance. For those cases, the id must be given as a lambda
taking the instance as argument:

handle :foo, with: :foo, id: ->(instance) { :"foo_#{instance.object_id}" }

Lastly, we force identifiers to be a Symbol.