add Format.pp_print_nothing

[zapashcanon]

Hi,

This adds a Format.pp_print_nothing. I noticed I'm often defining this function when I'm starting to have too much lines starting with Format.pp_print_list ~pp_sep:(fun _fmt () -> ()).

Format.pp_print_list ~pp_sep:Format.pp_print_nothing is not shorter - unless you have Format opened where you gain 3 characters :-) - but it makes code easier to read.

[gasche]

The rationale makes sense and the cost of the change is very low. Approved.

This is a stdlib PR so a second approval is needed.

[nojb]

LGTM. Changes entry and @since annotation in the .mli needed.

[zapashcanon]

Thanks, should be good now.