Make country functions const

[mplanchard]

We have a use case where we'd like to maintain a static list of
countries to check some values against, e.g.:

const COUNTRIES_TO_CHECK: &[Country] = [
      Country::the_united_states_of_america(),
      Country::canada(),
];

We could just use the 3-letter codes, but it would be nice to have the
entire Country object for more flexibility (e.g. to check against
either 2-letter or 3-letter inputs), and without needing a workaround
like:

COUNTRIES_TO_CHECK
    .map(|code| Country::from_code(code).expect("known valid country"))
    .any(|country| country.alpha2 == val || country.alpha3 == val)

This PR updates the country and lookup macros to enable const
construction of Country tables, so that country construction can be
used in const contexts.

---

In a second commit, I have updated the const functions to use inline
const blocks. This will guarantee that country resolution is always
evaluated at compile time, allowing the cost to be ~zero at runtime,
even outside of const contexts. This requires Rust 1.79, so I have
added a rust-version property to Cargo.toml to indicate the MSRV
(released June, 2024). If you'd prefer to not add a feature requiring
an MSRV, let me know and I can drop that commit.

[mplanchard]

Ah, the LazyLock invocations already required Rust 1.80, so I'll update the MSRV to that.

[mplanchard]

Note that this also paves the way to making all of the lookup functions reliant on match statements rather than lazy static hashmap lookups.

That could go either way, with values either mapping to e.g. "us" => Some(const { Country::the_united_states_of_america() }), or we could define consts for every country as part of the country! macro, which would allow like "us" => Some(Self::THE_UNITED_STATES_OF_AMERICA). The latter would be preferable if you'd be interested in exposing the consts in the public interface. Otherwise it's 6 of one, 1/2 dozen of the other.

If you're interested in this as a followup, lmk and I'll open a PR.

[mikelodder7]

Yeah I agree. Its time to update this.