Add a CallbackPair handler implementing Drop and IntoCallbackReceiverPair

[JEnoch]

This struct holds 2 callbacks functions:

• callback: the typical callback function. context will be passed as its last argument.
• drop: a callback called when this handler is dropped.

It can be passed to any subscriber or getter function accepting a callback.
E.g.:

• GetBuilder::with(Handler)
• SubscriberBuilder::with(Handler)

[p-avital]

@Mallets requesting your review in case you'd like to change a field name or two from the proposed ones :)