Make setImmediate.js more minifiable

Taken together, this series of commits reduces the uglified size of `setImmedate.js` from 3277 to 1563 bytes (52%) and cuts the uglified+gzip size from 1013 to 731 bytes (28%).

I made sure to isolate each logical change in its own commit, so if there are objections to any particular refactoring step, it can be tweaked or removed without interfering with other steps.