bug: remainder may return negative values

[brlewis@alum.mit.edu]

#|kawa:1|# (remainder 14853098170650239300 4000000000)
-1644727996
#|kawa:2|# (invoke 14853098166355272004 'longValue)
-3593645907354279612

I discovered this while trying to build a guile-compatible
pseudo-random number generator based on java.util.Random.