Fix bad uses of base and lbnd on rank 1 arrays

Fix bad uses of base and lbnd on rank 1 arrays

 * libguile/array-map.c
   - rafill, ramap, rafe, racp: object from SCM_I_ARRAY_V always
     has base 0, lbnd 0 and inc 1; make use of this.
 * libguile/arrays.c
   - array_handle_ref, array_handle_set: idem.
   - array_get_handle: sanity check.
 * libguile/generalized-vectors.c
   - scm_c_generalized_vector_ref, scm_c_generalized_vector_set_x:
     pos should be base when idx is lbnd. Furthermore, pos should be signed and
     have its overflow checked; do this by handling the job to
     scm_c_array_ref_1, scm_c_array_set_1_x.
 * libguile/generalized-vectors.h
   - fix prototypes.