In order for clients to include the library header-only, we should not have any external dependencies. As such, I would like to remove the usage of fmt in the library. We can keep it in the tests and examples.

Ideally we would replace it by std::format, but I remember GCC and Clang having some issues with their implementations in the std.

The goal of this issue is to investigate whether we can use std::format instead of fmt, and replace it if possible. If this is not (yet) possible, we should do the string concatenation manually.