[Nickle] Bugs using // with large negative dividend

[Carl Worth]

Here's some more mystic behavior of the // operator.

When the magnitude of a negative dividend exceeds some threshold, the
direction of the rounding changes:

	> a=-200000000;
	> b=-3;
	> a//b
	66666666
	> 10*a//b
	666666667

	> a=-200000000;
	> b=3
	> a//b
	-66666667
	> 10*a//b
	-666666666

Positive dividends do not appear to tickle the bug.

Also, results yielding a fractional part less than 0.5 seem to be
unaffected as well.

-Carl

PS. I don't know whether my attempt to validate the trapezoid
implementation using nickle is finding more bugs in the trapezoid code
or in nickle. ;-)