Introduce Operation trait

[pesterev]

Currently, all operations map to a prepared piece of code using the match construction. It's not ergonomic enough and not able to use queries.

My suggestion is to introduce the Operation trait inspired by this discussion:

trait Operation {
    fn execute(&mut self, status: Arc<Status>, client: Client, index: usize, test_env: TestEnv);
}

And to implement it for all operations. The execute will accept a client instance and use it to make queries and submissions. Anyway, the design of the trait is discussable.

UPDATE: Let's keep Operation impls separately in different modules/files.