Build example programs to avoid breaking them on API changes
[C++20] Clean up using-declarations. Never write a using-declaration before #including another header file!
Make C++ files in doc/examples/ compile again.
Make ample use of the contextual keyword 'override'. This patch adds the C++11 contextual keyword 'override' to every overridden virtual function declaration except where that would incur macro duplications. Along the way, it fixes some comments about member functions 'virtuality'.
tutorial: simplify the instructions on writing extension classes. Don't mention (un)archiving while describing the `mystring' class (the minimalistic extension class), so readers have less chances to get confused.
Don't force every algebraic class to implement archiving/unarchiving. So people who don't use (un)archiving don't need to bother with it.
Wipe out remnants of custom RTTI. Custom RTTI considered harmful, final part.
Added example for mystring class [Sheplyakov].