refactor: change schema version hint to unordered_map

include/jvalidate/detail/reference_manager.h

@@ -110,7 +110,7 @@ public:
    * vocabularies, else a user-schema is loaded.
    */
   Vocabulary<A> const & vocab(URI const & schema) {
-    static std::map<std::string_view, schema::Version> const schema_ids{
+    static std::unordered_map<std::string_view, schema::Version> const schema_ids{
         {"json-schema.org/draft-03/schema", schema::Version::Draft03},
         {"json-schema.org/draft-04/schema", schema::Version::Draft04},
         {"json-schema.org/draft-06/schema", schema::Version::Draft06},