Nix compilation is getting quite slow. Maybe we can speed it up with some header discipline. Some offenders:

• #include <regex> adds about 0.4s per compilation unit.
• #include <nlohmann/json.hpp> adds about 0.7s per compilation unit.
• The 2 boost headers included in fmt.hh (<boost/format.hpp> and <boost/algorithm/string/replace.hpp>) add about 0.7s to every compilation unit. Not easy to get rid of, but maybe we can switch to C++20's std::format or https://github.com/fmtlib/fmt.