Replace "Empty" with no-field output structs

[adamchalmers]

Right now, commands like Extrude that return no data will return the () or "unit". It's basically a struct with no fields. In the big enum of all response types, there's a variant Empty which corresponds to this.

But in the future, if we change this command to return some data, it'll be a breaking change because it changes the type of a response.

Suggested improvement: every modeling command should have an output struct, e.g. output::Extrude. If it has no data, that output struct will just have no fields. Then, if we add response data in the future, we can just add that field.

This will simplify the design of the crate because there'll be no need for a special Empty variant that works differently.