Macros for concise precondition checks

[anorth]

We have lots of code like this:

                        if ids.len() != 1 {
                            return Err(actor_error!(
                                illegal_argument,
                                "expected 1 allocation ID, got {:?}",
                                ids
                            ));
                        }

Write a couple of macros so we can write

require_argument!(ids.len() == 1, "expected 1 allocation ID, got {:?}", ids)

and similar for state and "forbidden"-type errors. (Macros are necessary for var-args)

[Stebalien]

Note: for fixed-length parameter arrays, we should probably just be using arrays. But otherwise, yeah, require_argument would be nice.