When writing a fasl stream, add extra graph points as needed
to limit recursive reading to about 500 non-tail recursions.
original commit: a6759efdc6ac68e45ec8755a5fe9b75038e173a5
Revert the use of ephemeron pairs in weak hashtables, since the
difference is visible via guardians. Add hashtable based on ephemerons
(to avoid key-in-value problems) as an explicit variant.
original commit: 31ac6d78592e1a9ba6bfbe802260e3d56d4cf772
