Fix intDiv for bigints by not using floats

[MonoidMusician]

I tested that this agrees with behavior of the JS Prelude for negative integers

unfortunately both purerl and purescript-backend-erl will optimize to regular Erlang div (in different scenarios, since backend-erl does optimizations), so the correctness of this FFI implementation ends up not mattering much

(how I found the bigint bug was by disabling the optimization)

Proof that this works for bigints:

3> 80126697530880000000000 / 55319040000000000.
1448446.9999999998
4> intDiv(80126697530880000000000, 55319040000000000).
1448447