Related to #1737

After removing dependency on Vorpal, we'll be able to further refactor the command code and remove the CommandInstance construct. It will help us simplify code and tests and remove obsolete code.

If I understood it correctly, Vorpal requires CommandInstance to store information about the current command in its execution context. Since out setup is easier and the context doesn't span beyond the execution of the single command, we can access all the necessary information from the command itself without having to copy it to the context.

I'll pick up this refactoring, after we closed #1737.