1.0.4.84: better EQUAL hash-tables, revert 0.9.15.21 (non-compliant SXHASH)

        * The value of SXHASH on non-string/bitvector arrays should not
          change when the contents of the array change (clhs sxhash,
          clhs 18.1.2.2). So the commit in 0.9.15.21 is invalid.
        * But luckily nothing requires us to use SXHASH for our own
          tables. Change the definition of EQUAL-HASH to punt to EQ-HASH
          for data types where EQUAL == EQL