The Artifact instance within Element is now initialised when the
relevant keys are available, with the weak & strong keys being
in the constructor. It also introduces the notion of a strict_artifact,
which is used to make a distinction of the Artifact keys and cached
state dependant on the context defined strictness. The keys are updated
when a relevant key change occurs. As the keys are now internalised
within Artifact, the majority of key determinsim and parameterisation
in public Artifact method calls are now redundant.