add SCM_{PACK,UNPACK}_POINTER

add SCM_{PACK,UNPACK}_POINTER

* libguile/tags.h (SCM_UNPACK_POINTER, SCM_PACK_POINTER): New macros.
  The old SCM2PTR and PTR2SCM were defined in such a way that
  round-tripping through a pointer could lose precision, even in the
  case in which you weren't interested in actually dereferencing the
  pointer, it was simply that you needed to plumb a SCM through APIs
  that take pointers.  These new macros are more like SCM_PACK and
  SCM_UNPACK, but for pointer types.  The bit representation of the
  pointer should be the same as the scm_t_bits representation.

* libguile/gc.h (PTR2SCM, SCM2PTR): Remove support for (old) UNICOS
  pointers.  We are going to try tagging the SCM object itself in the
  future, and I don't think that keeping this support is worth its
  cost.  It probably doesn't work anyway.

* libguile/backtrace.c:
* libguile/bytevectors.c:
* libguile/continuations.c:
* libguile/fluids.c:
* libguile/foreign.c:
* libguile/gc.h:
* libguile/guardians.c:
* libguile/hashtab.c:
* libguile/load.c:
* libguile/numbers.c:
* libguile/ports.c:
* libguile/smob.c:
* libguile/strings.c:
* libguile/symbols.c:
* libguile/vm.c:
* libguile/weak-set.c:
* libguile/weak-table.c:
* libguile/weak-vector.c: Update many sites to use the new macros.