accesslib: add support for multi-enrolments

accesslib: add support for multi-enrolments

I had dropped support for multi-enrolments on the same context.

 Oops!

This patch reinstates it, changing the semantics of the 'ra'
array leaves from an int to an array of ints.

So no instead of

   $USER->access['ra']['/1/12/543'] = 5

we have

   $USER->access['ra']['/1/12/543'] = array(5)

the functions that build the array, and the array walkers have
been updated. This touches...

  Writing RAs
    load_all_capabilities()
    get_user_access_sitewide()
    get_user_access_bycontext()

  Reading RAs
    has_cap_fromsess()
    aggr_roles_fromsess()

Thanks to Matt Clarkson for mentioning multi-enrolments!