fix: Actually avoid serde as dependency, when the serde feature is off

[bpeetz]

faster-hex has serde as default dependency, and was not marked with default-features = false.

I found this while using gix with it's default features turned off.

[Byron]

Thanks a lot, a great catch!

I should remember to always check for default features, and maybe even protect from future added defaults by always disabling default features in third-party dependencies.

[bpeetz]

Thanks for the quick merge!