Add Python-like set operations to flat_set

[AntoinePrv]

• Add flat-set doctest printer
• Add Python-like set operations to flat_set

[Klaim]

I feel like it's not a good idea to wrap generic algorithms into member functions for convenience because

• as soon as we switch to C++20 we will get ranges, making these functions obsolete or equivalent;
• usually these algorithms are defined as free functions and/or have different names in the c++ world (also see: https://www.boost.org/doc/libs/1_82_0/doc/html/boost/container/flat_set.html and the version that was voted in C++23: https://en.cppreference.com/w/cpp/header/flat_set )

I'm not completely against adding these functions but could you make them free instead of member at least?

[AntoinePrv]

The algorithms exists as member functions because they can leverage the specific internals of the class for better algorithmic complexity. Just like we don't use std::find on a map.

IMHO C++ <algorithm> header is confusing. There are "range manipulation" algorithms (filter, transform...) that are in fact generic and the C++20 range library does amazingly well. Then there are more complex algorithms (e.g. non-linear), and these cannot be decoupled from data structure. I think set operations fall in the second category.

[JohanMabille]

nitpicking: I think we can simply use flat_set in the declaration and implementation, the template parameters will be deduced.

[JohanMabille]

🥲