Rewrite expression templates and type list manipulation with pack indexing

[mpusz]

https://en.cppreference.com/w/cpp/language/pack_indexing

[burnpanck]

Nice! Though currently, that's only Clang 19, and no GCC yet, according to cppreference.com.

[JohelEGP]

Boost.Mp11 should opt into the newer features.
But we don't use Boost.

[mpusz]

This is why no milestone has been assigned to this. I created this mostly to remember that we should do it and provide it conditionally only on the platforms that support it. Hopefully, the compile times will improve.