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
7.ss, scheme.1.in, comments of many files original commit: 06f858f9a505b9d6fb6ca1ac97234927cb2dc641
original commit: 47a210c15c63ba9677852269447bd2f2598b51fe