Add custom to_json and from_json functions for ExperimentalFeature

nix show-config --json was serializing experimental features as ints.
nlohmann::json will automatically use these definitions to serialize
and deserialize ExperimentalFeatures.

Strictly, we don't use the from_json instance yet, it's provided for
completeness and hopefully future use.

src/libutil/experimental-features.hh

@@ -51,4 +51,11 @@ public:
     MissingExperimentalFeature(ExperimentalFeature);
 };
 
+/**
+ * Semi-magic conversion to and from json.
+ * See the nlohmann/json readme for more details.
+ */
+void to_json(nlohmann::json&, const ExperimentalFeature&);
+void from_json(const nlohmann::json&, ExperimentalFeature&);
+
 }