new annotation or pragma to mark functions that are intended to be only used during compile time

[Ilya]

Hey, I did some measurements with my initial implementation and 
even marking the obviously CT-only things in our codebase gives 
~7% compilation time improvement for some of the object files. 
That's not super exciting, but still something we'd like to get 
(we are annoyed by the long compilation times a lot).

What are the next steps? This thread is super quiet... Does that 
mean there is not enough interest in this? Should I try 
proceeding with the DIP PR? Or it doesn't make much sense?