List of things I learned (or refreshed) with this year's AoC.

* Day 1: std::count()
* Day 2: std::sstream
* Day 3: std::string_view
* Day 4: nothing special, some practice with classic and a weird trick
  to assign two variables at once with a pair of references pair<int&, int&>
* Day 5: std::find and std::replace
* Day 8: set::insert_range(), but it is from C++23 only
* Day 9: std::views
* Day 11: if(init; cond)
* Day 19: more practice with std::string_view for constant string
* Day 22: std::views::values for map (and keys)
* Day 24: std::fill