Composable Activities

[johthor]

Is your feature request related to a problem? Please describe.
#7 showed that there activities that don't fit into the simplistic subject predicate object [post target] schema.

Describe the solution you'd like
Activities should be composable like activity(step, subject, predicate, compose(object, post, compose(target, even, more))).

And if possible also be able to describe splits in the activity chain.