* libguile/weak-set.c:
* libguile/weak-set.h: New files, implementing weak sets, for use in the
symbol table and port set. Eventually we will be able to remove weak
pairs.
* libguile.h:
* libguile/Makefile.am: Add new files.
* libguile/evalext.c:
* libguile/gc.c:
* libguile/init.c:
* libguile/print.c:
* libguile/tags.h: Add support for the new types.
