Appease Clippy and get automagic enum to string

Using https://github.com/Peternator7/strum

Cargo.toml

@@ -26,6 +26,7 @@ sentry-tower = { version = "0.32.0", features = ["http", "axum-matched-path"] }
 sentry-tracing = "0.32.0"
 serde = { version = "1.0.188", features = ["derive"] }
 serde_json = "1.0.107"
+strum_macros = "0.26.2"
 tokio = { version = "1.32.0", features = ["macros", "rt-multi-thread"] }
 tower = "0.4.13"
 tower-http = { version = "0.5.0", features = ["catch-panic", "trace", "fs", "compression-full"] }

src/wanikani.rs

@@ -15,24 +15,15 @@ pub struct WaniKaniAPIClient<'a> {
     client: &'a reqwest::Client,
 }
 
+#[derive(strum_macros::Display)]
+#[strum(serialize_all = "snake_case")]
 enum SubjectType {
     Radical,
     Kanji,
     Vocabulary,
     KanaVocabulary,
 }
 
-impl ToString for SubjectType {
-    fn to_string(&self) -> String {
-        match self {
-            SubjectType::Radical => "radical".to_string(),
-            SubjectType::Kanji => "kanji".to_string(),
-            SubjectType::Vocabulary => "vocabulary".to_string(),
-            SubjectType::KanaVocabulary => "kana_vocabulary".to_string(),
-        }
-    }
-}
-
 const APPRENTICE_SRS_STAGES: [u8; 4] = [1, 2, 3, 4];
 
 impl<'a> WaniKaniAPIClient<'a> {