Extracting cell code to gc.h

[Dirk Herrmann <dirk at ida dot ing dot tu-bs dot de>]

Hi!

Currently pairs.h contains a lot of macros that deal with cells, but which
not really are specific to scheme pairs.  I suggest to move these macros
to gc.h.  The same is true for the SCM2PTR / PTR2SCM macros, which are
also just cell pointer related.

There are some points in favor of such a move:
* It's cleaner.  Cells are a more general thing than pairs.
* Everybody that uses cells automatically has to include the pairs
  stuff.  After this move, gc.h will still be included everywhere, but
  pairs.h will only have to be included by code that actually uses pairs,
  and not just cells.
* Since, according to the recent discussion about variables of type SCM,
  macros like SCM_CAR and SCM_CDR should in future only be used for real
  scheme pairs, these will be kept in pairs.h, thus emphasizing the
  separate concepts.

Best regards
Dirk Herrmann