logilab-common #98742 make it easy to store instances in registries, not only classes [done]

it is useful for at least http://www.cubicweb.org/ticket/2406609

prioritynormal
typeenhancement
done in0.59.0
load0.500
load left0.000
closed by#dc5f7cbd3134 [registry] introduce RegistrableObject and RegistrableInstance base classes. Closes #98742, #99757eea1647 [registry] introduce objid/objname method on registries instead of using function/inlined code. Partially closes #98742