* Guile Weak References

This is a simple little library that makes use of the weak vector functionality in core Guile
to implement weak references, objects which weakly point to other objects.

The interface is very simple:

(make-weakref obj)
- Create a weak reference, which will not count as a reference, so the object can be garbage collected

(weakref-object ref)
- Get the object associated with the reference, or #f if it has been garbage collected.

** Implementation
This is a very simple implementation (thanks David Thompson for a simpler idea). It simply calls a single-
element weak vector a weak reference. Initially this used the weak hash table support but using a weak
vector is far simpler and uses less memory.