Spike for consider using `gen_statem`

[joaomdmoura]

Erlang has gen_statem a generic state machine behavior. Using it behind the scenes could be a good thing to enable ppl to use Machinery on a broader set of applications, but it's also a turning point that you push Machinery into a different direction. Right now it looks more like an Ecto extension, just a thin layer people could use on top of a struct, nothing fancy and I kind like that better, but still think a spike here could have a good use.