Consider replacing hard-coded country_code mappings with external Rust crate

[rhigman]

The country_code enum added under #314 is a bespoke list of ISO 3166-1 alpha-3 codes with string conversions to English country names, but a more robust solution would have been to check if any existing Rust crates handle this mapping. Unsurprisingly, some do exist, e.g. https://docs.rs/iso3166-1.

This would require less manual updating as the ISO 3166-1 list (inevitably) changes, although it would give us less control over the representation of politically sensitive (or just overlong/over-formal) country names. I'm also not sure how (if) it would work for defining the database enum.