[MERGE-REQUEST] Bugfix in `guile-wct.c'; support for `mark' and `cleanup'

[Ludovic Courtès]

Hi again,

Could you please merge the following two patches:

  patch-5
      Fixed `guile-wct.c' so that the user-defined mark function gets called.
  patch-6
      Added support for `mark' and `free' in <gw-wct>.

The first one fixes a bug in `gw_wct_create ()':  the MARK argument was
just ignored instead of being assigned to TYPE_DATA->MARK.
Additionally, it updated `wcp_data_mark ()' so that TYPE_DATA->MARK is
called when non-NULL.

The second patch proposes the addition of two slots to the <gw-wct>
class, namely `wcp-mark-function' and `wcp-free-function', that allow
the user to specify a pointer to a GC mark function and to a free
function.  `initializations-cg', in guile/g-wrap/guile.scm, is modified
accordingly to pass those pointers to `gw_wct_create ()'.

This second change might be somewhat controversial since it is partly
Guile-specific.  I believe `wcp-free-function' is _not_ specific,
but the other one is:  not all garbage collectors use a ``mark and
sweep'' algorithm.  However, I can't think of any other way to do that.

Thanks,
Ludovic.