Library for weak data structures.
Returns a new ephemeron. This ephemeron holds a weak reference to
key, such that value will only be traced by the GC if
key is referenced from an external object.
Returns true iff x is an ephemeron.
Returns true iff ephemerons key has been GCed.
Returns ephemerons key, or #f if it has been GCed.
Returns ephemerons value.