Avoid an flabs call in flonums-between

[pavpanchekha]

I feel dumb I didn't see this the first time! The flonums-between function is optimized to allocate less, but it still calls flabs to normalize positive and negative zero before extracting the high and low words of a flonum. And flabs allocates because Chez flonums are heap-allocated. But we don't need to call flabs! We can just not extract the sign bit!

[mflatt]

FWIW, it seems wrong that the flabs result here was not unboxed. The problem is that only one argument to flbit-field is a flonum, and so it needs more special treatment (along the same lines as flvector-set!). I'll fix that at the Chez Scheme level, independent of this change.

[pavpanchekha]

Good point, but either way this seems like a good change… Worst case it’s only a (small) perf and simplicity improvement.