dead_code warnings on various slides

[fw-immunant]

Now that we see stderr, warnings for dead code are visible on quite a few slides. In just about every case we do want the dead code to be present because it has didactic value. We should add #![allow(dead_code)] where appropriate or pass the corresponding flag to rustc if the sandbox allows that.

[djmitche]

We noticed this during instruction this week as well. I'll see if we can fix it up. Another option may be to include it but hidden (some slides appear to have hidden code, and I'm not sure how that works!)