new lint: single_char_lifetime_names

This pull request adds a lint against single character lifetime names, as they might not divulge enough information about the purpose of the lifetime. This can make code harder to understand. I placed this in `restriction` rather than `pedantic` (as suggested in #8233) since most of the Rust ecosystem already uses single character lifetime names (to my knowledge, at least) and since single character lifetime names aren't incorrect. I'd be happy to change this upon request, however. Fixes #8233.

- [x] Followed lint naming conventions
- [x] Added passing UI tests (including committed `.stderr` file)
- [x] `cargo test` passes locally
- [x] Executed `cargo dev update_lints`
- [x] Added lint documentation
- [x] Run `cargo dev fmt`

changelog: new lint: [`single_char_lifetime_names`]

clippy_lints/src/lib.rs

@@ -351,6 +351,7 @@ mod self_named_constructors;
 mod semicolon_if_nothing_returned;
 mod serde_api;
 mod shadow;
+mod single_char_lifetime_names;
 mod single_component_path_imports;
 mod size_of_in_element_count;
 mod slow_vector_initialization;
@@ -858,6 +859,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(needless_late_init::NeedlessLateInit));
     store.register_late_pass(|| Box::new(return_self_not_must_use::ReturnSelfNotMustUse));
     store.register_late_pass(|| Box::new(init_numbered_fields::NumberedFields));
+    store.register_early_pass(|| Box::new(single_char_lifetime_names::SingleCharLifetimeNames));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }