Debug logging I can compile-away in release

[dbuenzli]

@WardBrian

The motive isn’t binary size, it’s avoiding (possibly 10s of thousands) of calls to a function that looks like let debug_log x = if !debug then print_endline x else (), where x could be some not-cheap-to-evaluate string.

You might be interested in the scheme used by Logs for logging, see the links in this message.