Fix comparision of floating point infinities and NaNs with ratios/bignums

Fixes a bug where statements such as

(= 1/2 ext:double-float-positive-infinity)

throw an arithmetic-error, when they should just silently return nil.

Also fixes the sign of the infinite floating point returned by e.g.:

(expt 0.0 -1.0)

(this unrelated change is in my opinion too small to justify an extra merge request for)