Try avoiding unnecessary copy constructions

[SchrodingerZhu]

when handling strings, the library makes quite a lot extra copy constructions. Some basic improvements:

• when passing string by value in constructors, please use std::move before assign it to the class field.
• please notice that for (std::string i : container) will copy-construct a string in each iteration. There are many cases in the library (for example, see system/exception.h) that using a const reference is enough. As the library is to be shown to students, I guess better practice is needed to apply here.
• string_view can be used if you just want to access some slices (C++17).