rustdoc: Clean up handling of lifetime bounds

Previously, rustdoc recorded lifetime bounds by rendering them into the
name of the lifetime parameter. Now, it leaves the name as the actual
name and instead records lifetime bounds in an `outlives` list, similar
to how type parameter bounds are recorded.

src/rustdoc-json-types/lib.rs

@@ -323,7 +323,7 @@ pub struct GenericParamDef {
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum GenericParamDefKind {
-    Lifetime,
+    Lifetime { outlives: Vec<String> },
     Type { bounds: Vec<GenericBound>, default: Option<Type> },
     Const { ty: Type, default: Option<String> },
 }