Create a Default impl for ProptestStatefulConfig

[nickelization]

Would be nice if we could just call ProptestStatefulConfig::default() if we don't want to have to fill in all the struct fields by hand. Maybe that gives us min ops of 10 and max of 20 or something, and a default timeout of 60 seconds for async tests?

It'd be even nicer if we had extra methods like default_with_timeout(Duration) and default_with_case_size(range).