accesslib: Draft get_context_users_byrole()

accesslib: Draft get_context_users_byrole()

 Draft - use for the course participants list page

 Uses 1 DB query (cheap too - 2~7ms).

 TODO:
 - implement additional where clauses
 - sorting
 - get course participants list to use it!

 returns a users array, both sorted _and_ keyed
 on id (as get_my_courses() does)

 as a bonus, every user record comes with its own
 personal context, as our callers need it straight away
 {save 1 dbquery per user! yay!}