low-level RNG interfaces deal in scm_t_uint32, not unsigned long

low-level RNG interfaces deal in scm_t_uint32, not unsigned long

* libguile/random.h (scm_t_rng): random_bits returns a scm_t_uint32.
  (scm_i_uniform32, scm_t_i_rstate): Internal RNG returns a
  scm_t_uint32, as advertised, instead of unsigned long.
  (scm_c_random): Return a scm_t_uint32 instead of an unsigned long.

* libguile/random.c (scm_i_uniform32, scm_i_init_rstate_scm):
  (scm_i_expose_rstate, scm_c_random, scm_c_random_bignum, scm_random)
  (scm_init_random): Adapt types to match implementation.