Add default function implementations for the ModelState trait

[nickelization]

Currently you have to implement every function in ModelState, even if you don't care about things like post-test cleanup or postconditions. It's probably worth making default no-op implementations of those functions so that tests can skip implementing ones that they don't need. (For an example of a case like this, see tests/counter.rs.)

Note, however, that I specifically do not want to add default implementations for preconditions_met; we could easily add a default function that returns true, but I think it would be unusual for a real-world test to not care about preconditions, and if you don't specify any then the test may appear to work fine until you find a failure and get bad results from shrinking. As such, I think it 's important to be explicit about it if you really don't want to define any preconditions. Similar logic applies to next_state, in my opinion.

As such, the functions I'm envisioning default implementations for are:

• check_postconditions
• clean_up_test_run
• init_test_run (though we might need to change the API to require Default on ModelState::RunContext in order to be able to make a sane default implementation for this one, and then that forces us to implement Default on all run context types which seems excessive and annoying...so maybe not? TBD)