support unsafe_ref/assign for any isbits type, including pointer-free immutable composites
add pointer_from_objref (inverse of unsafe_pointer_to_objref)
update manual for pointer_from_objref and to note limitations on unsafe_assign
Merge pull request #2515 from stevengj/unsafe_immutable

RFC: extend unsafe_assign to work for any isbits type, add pointer_from_objref