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
to simplify ($fxu< (most-positive-fixnum) e) => (fx< e 0) so we
don't have any incentive in special casing length checks where
the maximum length happens to be (most-positive-fixnum).
5_4.ss, 5_6.ss, bytevector.ss, cmacros.ss, cp0.ss, cpnanopass.ss,
mkheader.ss, primdata.ss, prims.ss,
fasl.c, gc.c, types.h
root-experr*, patch*
original commit: 9eb63deda025fd4560b54746b21a881c01af46d6
