refactored code, made new comments, extended manualtest.cpp, commented

out tests of unimplemented stuff in test.cpp

String.h

@@ -1,5 +1,7 @@
 #pragma once
 #include <iostream>
+#include <cstring>
+#include <memory>
 #include "StringValue.h"
 #include "Char.h"
 class Char;
@@ -16,6 +18,9 @@ private:
 
     StringValue* _str;
 
+    char* copy_and_concat(const String& str) const;
+    char* copy_and_concat(char c) const;
+
 public:
 
     String() noexcept;