rbac2: rbac enforcer rewrite
- use repo AT-URI as identifier
- use `dom` field rather than `obj` to filter by repository
So now it's "user with role A in repo B can do action D to object C"
where `A,B,C,D` are `sub,dom,obj,act`.
Many parts are missing, but should be enough to test this new RBAC
enforcer package in spindle.
Related issue: <https://tangled.org/tangled.org/core/issues/282>
Signed-off-by: Seongmin Lee <git@boltless.me>