Make Virtus warning-free

This makes Virtus warning-free.

It would be great if Virtus was warning-free for the sake of other projects using it; some test suites are ran with warnings on (to catch things like unused variables and misspelt ivars), and it would be great if Virtus played along. :)

There are three commits because they deal with two different warning types and handle one of them in two different ways; I’m happy to adjust the fixes as you see fit (and squash the PR if that’s what you prefer).

Let me know if you want me to come up with a test that keeps Virtus warning-free (I’m not sure whether it’s worth the hassle, but I’ll be happy to provide it if you want).