[MERGE-REQUEST] More robust implementation of `aggregated'

[Ludovic Courtès]

Hello, and happy new year G-Wrappers!  ;-)

As you might have seen on `guile-devel'[0], relying on object properties
to implement `gw_wcp_set_dependencies ()' (used for the `aggregated'
typespec[1]) was not a good idea, given the sloppiness of the GC
semantics in that area (at least, in the absence of a patch similar to
the one I proposed).

So I re-implemented it, in `guile-wct.c', in (i) a more straightforward
manner and (ii) in a way that doesn't rely on source properties and is
consequently more robust.  This works by adding a `dependencies' field
(containing a list of Scheme objects) to the `wrapped_c_pointer_data'
structure.

Afterwards, it occurred to me that the `scm_data' field was intended to
be used for the very same purpose.  Hence the second patch.  The third
patch adds a little bit of documentation on that topic.

  patch-16
      Re-implemented `gw_wcp_set_dependencies ()' in a more robust way.
  patch-17
      Removed the `scm_data' field from `wrapped_c_pointer_data' in 
`guile-wct.c'.
  patch-18
      Documentation: added a discussion of the `aggregated' type qualifier.

To summarize, the `aggregated' typespec functionality is now spread over
patches 7, 13, 16, 17, 18.

Andreas: can you please either merge this or speak out if you don't have
time to do that or if you think that this feature sucks and that you
don't want to merge it anyway?  :-)

More generally, there is a number of patches from me that you do not
have merged yet and I have no idea whether this is due to lack of time
or disapproval.  I would *really* appreciate if you could merge them, or
say something, or even both.  ;-)

In fact, once we've made some progress on those patches, I think it
would make sense to release something.  I believe that even
documentation alone would justify a release.  What do you think?

Thanks!

Ludovic.

[0] http://lists.gnu.org/archive/html/guile-devel/2005-11/msg00008.html
[1] http://lists.nongnu.org/archive/html/g-wrap-dev/2005-09/msg00006.html