New: SCM_DEBUG_CELL_ACCESSES

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

Hello list.

I added SCM_DEBUG_CELL_ACCESSES as a new debug option to guile.  It's not
a big patch, but already helped me to find a little strangeness in
guile:  Jump buffers got an extra cell allocated, but not on the heap.  I
changed it such that jump buffers now take up a double cell.  Otherwise,
everything else (i. e. the test suite) seems to run fine.  But, its
slooow.  Here's the documentation from __scm.h:

+/* If SCM_DEBUG_CELL_ACCESSES is set to 1, cell accesses will perform
+ * exhaustive parameter checking:  It will be verified that cell parameters
+ * actually point to a valid heap cell.  Note:  If this option is enabled,
+ * guile will run about ten times slower than normally.
+ */

Thus, developers now can detect if their code somehow passes
non-cell-but-looking-like-a-cell scheme objects.  I don't think that it is
a common problem, but providing this was simple.  Oh, and by the way I
fixed the scm_cellp function which was _always_ returning 'false' up to
now.  Looks like nobody has ever used it.

Have a nice weekend.
Dirk