Enhance tar_map() usage

[psychelzh]

See ropensci/targets#1375. After targets 1.9.0, there is no need to convert integer64 to character. Some example lines could be simpler:

tarflow.iquizoo/R/targets.R

Line 130 in 05ef92e

as.character(unique(contents$project_id)),

tarflow.iquizoo/R/targets.R

Lines 217 to 218 in 05ef92e

	project_ids <- as.character(unique(contents$project_id))
	game_id <- as.character(unique(contents$game_id))

[psychelzh]

Actually, I have turned to tar_eval(), so it is not relevant now.