(Possibly) provide a unified interface for transitions #145
Description
As discussed in TuringLang/Turing.jl#2304, it may be worthwhile implementing an AbstractTransition supertype in this library and defining a basic interface that exposes the key information in a transition.
Other libraries in the Turing ecosystem (e.g. Turing itself, AdvancedMH, AdvancedHMC) can then either directly subtype this or make a specialised implementation of Transition if necessary. Right now, all of those implement their own Transition types.