Add operation to generate CUE files.

[smimram]

Fixes #3608.

[toots]

I've been waiting for this! Thanks!

Couple of remarks:

• I think this should go on the extra lib
• Are you interested in extracting the time split logic into its own function (provided we don't have one already!)
• Can we add test to see what the file looks like?
• Can we add parameters to make it work with infinite sources? Like keeping the last n entries? Could be disabled by default.
• Can we make file change atomic? Otherwise you will run into situations where a partial file might be read. This happened with HLS outputs. You could factor out this code from output.hls:

          Fun.protect
            ~finally:(fun () -> try Sys.remove tmp_file with _ -> ())
            (fun () ->
              (try Unix.rename tmp_file filename
               with Unix.Unix_error (Unix.EXDEV, _, _) ->
                 self#log#important
                   "Rename failed! Directory for temporary files appears to be \
                    on a different file system. Please set it to the same one \
                    using `temp_dir` argument to guarantee atomic file \
                    operations!";
                 Utils.copy ~mode ~perms tmp_file filename;
                 Sys.remove tmp_file);

[smimram]

I think I have taken most of your suggestions in account. I propose that we merge as is and improve if usecases do show up...

[Moonbase59]

@smimram: Looks like you assume hours:minutes:seconds for the INDEX times, but they are actually MM:SS:FF (minutes, seconds, frames). Hydrogenaudio tells us there are 75 frames to a second: https://wiki.hydrogenaud.io/index.php?title=Cue_sheet#Most_often_used

[Moonbase59]

Frames are numbered 00…74.

[Moonbase59]

@toots @smimram Please check 50da0a3#r1448803161 (MM:SS:FF INDEX times).

[smimram]

Thanks for the comments, I am starting a bugfix session on #3624...