add weak table implementation

add weak table implementation

* libguile/weak-table.c:
* libguile/weak-table.h: New files, implementing open-addressed weak
  hash tables, similar to the implementation of weak sets.  This will
  let us remove weak pairs.

* libguile.h:
* libguile/Makefile.am:
* libguile/evalext.c:
* libguile/gc.c:
* libguile/init.c:
* libguile/print.c:
* libguile/tags.h: Update all the pieces for the new files and tc7.