convert internal weak hash table users to use the weak table api

convert internal weak hash table users to use the weak table api

The weak table API isn't public yet.  It could be after some review.
But we can go ahead and use it now internally.

* libguile/foreign.c:
* libguile/goops.c:
* libguile/objprop.c:
* libguile/procprop.c:
* libguile/smob.c:
* libguile/srcprop.c: Update weak table users to new API.  No locking
  needed!