implement borrowed_or

Allow the usage of `or` without consuming self. This can be useful
for pipelines that maintain some sort of owned state.

parser/src/yaml.rs

@@ -400,6 +400,16 @@ impl Yaml {
             this => this,
         }
     }
+
+    /// See `or` for behavior. This performs the same operations, but with
+    /// borrowed values for less linear pipelines.
+    #[must_use]
+    pub fn borrowed_or<'a>(&'a self, other: &'a Self) -> &'a Self {
+        match self {
+            Yaml::BadValue | Yaml::Null => other,
+            this => this,
+        }
+    }
 }
 
 #[cfg_attr(feature = "cargo-clippy", allow(clippy::should_implement_trait))]