Allow exposing of random number generator state

Allow exposing of random number generator state

Now the random number generator state can be obtained in external
(i.e. `read'/`write'-able) form via the new procedure
`random-state->external'.  An externalized state can be reinstantiated by
calling `external->random-state'.

* libguile/random.c (scm_i_init_rstate_scm, scm_i_expose_rstate): New
  internal functions.
* libguile/random.c (scm_c_make_rstate_scm, scm_external_to_random_state,
  scm_random_state_to_external): New public functions.
* libguile/random.h: Add prototypes for the above functions.

* libguile/random.h (scm_t_rng): Add new fields `init_rstate_scm' and
  `expose_rstate'.
* libguile/random.c (scm_init_random): Initialize the new fields in
  `scm_the_rng'.