Fix build warning, fail CI if build has warnings

[projectgus]

• Fix a build warning that snuck in with Added impl serde #67 (unused Result).
• Configure CI to fail if the build step produces any warnings.

[killercup]

I think we can get away without using a feature for this

[killercup]

The feature is a neat idea but why not just -D warnings?

[projectgus]

Good question... I originally implemented this by setting RUSTFLAGS=-D warnings and it looked like it built all of the dependencies with -D warnings too. (Then I copied the feature approach from here).

I didn't try passing -D directly to cargo though, I will try this and get back to you.

[projectgus]

Hey, sorry for the long turnaround on following this up!

• Can set RUSTFLAGS but this builds all crate dependencies with deny warnings too, which seems excessive.
• Can't pass -D warnings directly to cargo build, only to cargo rustc - but I think this has the same shortcoming as setting RUSTFLAGS.

The best summary I've found (with links to some other resources) is the thread linked in the previous comment https://users.rust-lang.org/t/how-to-fail-a-build-with-warnings/2687