Move shared rule pointer factories into individual rule files

src/rules/string.h

@@ -9,12 +9,14 @@ namespace tree_sitter  {
         public:
             String(std::string value);
             TransitionMap<Rule> transitions() const;
-            String * copy() const;
             bool operator==(const Rule& other) const;
             std::string to_string() const;
         private:
             const std::string value;
         };
+        
+        typedef std::shared_ptr<const String> string_ptr;
+        string_ptr str(const std::string &value);
     }
 }