Use model state

[seanmor5]

Resolves #372

[jonatanklosko]

@seanmor5 model_state = Axon.trace_init(model, input_template) this still warns because it expects the struct, should there be a function to build the struct from the map perhaps?

(we capture logs in most tests, so it doesn't show up there)

[seanmor5]

@jonatanklosko I updated trace_init to use Axon.ModelState.empty, and then pretty much everywhere in Axon we now use the Axon.ModelState.new to avoid warnings