Feature: Add transition_to!/3~4

[kyleboe]

Similar to other ! suffixed functions, the thought would be to either return the transformed struct or raise an error. The goal of this change would be to allow the transition_to! call to be chained in a pipeline.

Given:

defmodule MyStateMachine do
  use Machinery,
    states: ~w(pending completed),
    transitions: %{"pending" => ["completed"]}
end

defmodule MyStruct do
  defstruct [:state]
end

The spec for the function(s) may look something like this:

Machinery.transition_to!(%MyStruct{state: "pending"}, MyStateMachine, "completed")
# %MyStruct{state: "completed"}

Machinery.transition_to!(%MyStruct{state: "completed"}, MyStateMachine, "pending")
# raise Machinery.Errors.UndeclaredTransition