rstates point to rngs

rstates point to rngs

* libguile/random.h (scm_t_rstate): Include the rng in the rstate, so we
  can actually have multiple rngs. Instead of reserved0 / reserved1,
  reserve a double explicitly for scm_c_normal01.
  (scm_c_uniform32): Change to call the rstate's rng.

* libguile/random.c: Change to access an rstate's rng through its rng
  pointer.
  (scm_c_normal01): Instead of a flag and a next double, just check that
  the double is equal to 0.0. Excluding one value shouldn't affect the
  distribution, right?