Add iterative runner

[define-null]

This PR adds possibility to perform state machine testing with model that is aware of the system under test, as suggested in the https://github.com/proptest-rs/proptest/issues/378 I don't expect this code to be merged as is because while the approach is a fork with some modifications of the existing test_runner, there are some nuances here as well.

Some important changes additionally to iterative way of generating the transitions:

• additional pass to try to Delete once all the transitions have tested for shrinking
• when doing shrinking precondition check is less strict then when generating initial test. That's due to a good chance that operations will depend on each other, so as long as the test still fails after shrinking - droping more transitions because of the precondition is probably fine
• initial state shrinking is dropped here, as I considered that to be a bit problematic with the iterative runner, but if necessary that could be revisioned
• the approach that I picked doesn't change anything in the existing proptest library and could be potentially used as a standalone crate, but it will benefit from more tight integration, because of the way how the proptest runner functionality works.

Would love to hear some feedback about the PR

[define-null]

Thank you for spending time to reviewing this.

Why would someone use the SequentialRunner instead of just using the IterativeRunner with no checking of the SutState?

One of the use-cases for the existing SequentialRunner that I can see is when the developer has a well-defined self-contained reference model. If that's the case and one would like to have strong guarantees that model implementation is separate from the system under test - SequentialRunner would be prefered. Thus, I guess, the choice of the terminology - Reference, as reference model, rather than just the Model as in the IterativeRunner case.

But otherwise everything that one may want to implement using SequentialRunner would be possible to do with IterativeRunner as well.