Consider using less excessively large source files

[Eelis]

utilities.tex is 21K LOC. Source files this large cause some problems on github:

• when you browse to utilities.tex, the last 4 thousand lines are silently dropped
• when you make an issue whose description includes a reference to a line near the end of an extremely big source file, github doesn't turn it into a code citation as it normally does.

[jensmaurer]

See #3388

[jensmaurer]

The restructuring in #3388 has moved > 8k LOC to other files. Recent additions (e.g. std::format) have ballooned utilities.tex to 19k LOC again, but at least there's an improvement.

Closing for now.