Trait for collection

[Nutomic]

There are already traits for ApubObject, ApubActor and ActivityHandler for ease of handling the respective Activitypub types. There should be a similar trait for collections, which helps with pagination in particular.

[Nutomic]

A basic trait is implemented in #29, however it doesnt support pagination. I never had to use collections much, so I hope someone with more direct experience can contribute a good abstraction.