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?