Implement ephemerons.

Implement ephemerons.

Ephemerons are like weak pairs, except the datum is dropped if the
key is dropped, and references to the key through the datum don't
count if the only references to the datum are through the ephemeron.
In other words, the weak references to the key and datum are dropped
iff the key can be proven dead; conversely, the references to the key
and datum are preserved iff somebody else cares about the key.