Make _all_ return types alphabetical

Currently we're treating `null` and `false` as special cases, adding and removing them at various points in the process; this results in the return type list being `sorted(types-except-null) + null`. I want to remove the special-case magic, which will have the side effect of changing the return type to `sorted(types)`. I'm pulling that out into a separate PR because it makes the future PR easier to understand, and I also think it's valid to do this by itself

Author: shish