- Cleanups: My evil plot of making ex::bp private may finally be carried

- Cleanups: My evil plot of making ex::bp private may finally be carried
  out, provided one changes all the is_of_type(obj,type) to is_a<type>(obj)
  which we don't do right now because it would degrade performance on
  gcc-2.95.x.  Also, ex_to_type(obj) has gone for good now, we have been
  having ex_to<type>(obj) long enough.  ex_to_nonconst_type(obj) have gone
  as well, since they are almost never used and one can always cast aways
  the constness explicitly if need should arise.