Use a SmallString for the Yanked enum

[charliermarsh]

Summary

This is stored on File, which we create extensively. Easy way to reduce size.

[konstin]

Should we use a Option<Box<Yanked>> on File? This should be even smaller (usize) and yanked versions are rare.

[charliermarsh]

We should validate that that’s smaller, since SmallString is also a single word (but Yanked might be two now with discriminant).

[charliermarsh]

I can check and change if necessary before merging; no need for you to spend more time on it.

[konstin]

I've confirmed that that's the case on x86_64 with a test (below). The background is that even though Arc (and its variant such as ArcStr) are a pointer, and an x86_64 pointer only uses 48 bits of its 64 bits (leaving us 16 bit), rustc only sees a NonNull with a single niche (Null). Yanked needs at least two niches to represent Yanked::Bool, so it doesn't fit and we get padded to two words. In the experiments I made, all instances of enums where one variant contained a pointer and there was more than a single other variant without (like Option<T>), the enum would get padded to two words.

I don't feel strongly about what we do with File here, its 432 vs 424 bytes, this is for context.

    #[test]
    fn yanked_size() {
        assert_eq!(size_of::<Option<Yanked>>(), size_of::<usize>() * 2);
        assert_eq!(size_of::<Option<Box<Yanked>>>(), size_of::<usize>());
    }

    #[test]
    #[cfg(target_pointer_width = "64")]
    fn file_size() {
        assert_eq!(size_of::<Option<crate::File>>(), 432);
        #[derive(Debug, Clone, Deserialize)]
        #[serde(rename_all = "kebab-case")]
        pub struct File2 {
            // PEP 714-renamed field, followed by PEP 691-compliant field, followed by non-PEP 691-compliant
            // alias used by PyPI.
            pub core_metadata: Option<CoreMetadata>,
            pub dist_info_metadata: Option<CoreMetadata>,
            pub data_dist_info_metadata: Option<CoreMetadata>,
            pub filename: String,
            pub hashes: Hashes,
            /// There are a number of invalid specifiers on PyPI, so we first try to parse it into a
            /// [`VersionSpecifiers`] according to spec (PEP 440), then a [`LenientVersionSpecifiers`] with
            /// fixup for some common problems and if this still fails, we skip the file when creating a
            /// version map.
            #[serde(
                default,
                deserialize_with = "crate::simple_json::deserialize_version_specifiers_lenient"
            )]
            pub requires_python: Option<Result<VersionSpecifiers, VersionSpecifiersParseError>>,
            pub size: Option<u64>,
            pub upload_time: Option<Timestamp>,
            pub url: String,
            pub yanked: Option<Box<Yanked>>,
        }
        assert_eq!(size_of::<File2>(), 424);
    }

[charliermarsh]

I'll change separately.