fix floordiv and mod

[willow-ahrens]

Uses the new fld_nothrow and rem_nothrow functions in Finch to do floor division and remainder on sparse arrays with zeros without erroring.

[mtsokol]

LGTM! I enabled tests. I also added __abs__ to __mod__ function because in Python:

In [75]: -10 % 15
Out[75]: 5

where in Julia:

julia> -10 % 15
-10

[hameerabbasi]

Ouch -- __abs__ was a mistake, imho. We should add the divisor instead if the result is negative.

[willow-ahrens]

got it, there's a fix on its way