add weak sets

add weak sets

* 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.