changes

added id field to each exercise returned by exercise-download output type
serialize some dates returned by the TMC API as datetimes instead of strings (for example, CourseData::refreshed_at)
restructured the tmc-langs API to reduce nesting (use tmc_langs::CourseData instead of use tmc_langs::data::CourseData)...
download-or-update-course-exercises now downloads the latest submission by default if one exists and the exercise doesn't exist on disk. Added new flag download-template to force downloading the template in every case
removed download-or-update-exercises
renamed migrate to migrate_exercise
renamed LocalExercise to ProjectsDirExercise
added get_projects_dir
fixed update-exercises
fixed regression in prepare-stub

etc