Modelator v0.2.0

• provide two top-level functions to test a system using execution traces coming from TLA+ (see #44)
  • run_tla_steps will run anything that implements a StepRunner trait: this is suitable for small specs and simple cases
  • run_tla_events will run an implementation of EventRunner, which expects that a TLA+ state is structured, and contains besides state, also the action to execute, as well as the actionOutcome to expect.
• make Apalache the default model checker
• execute model checkers in a temporary directory (see #48)