Change randomness generator slightly

The comments on our proposal suggested:
  Be very careful with sampling from a uniform distribution over a range other
  than [2^n, 2^{n+1}). Floats are complicated.

Scheme uses a subtract-with-carry RNG. I couldn't find much literature online
about how uniform this is, but I think it's probably safe to assume that
(random 1.0) is uniform.