Use string and symbol accessors in struct, throw, and array funcs

Use string and symbol accessors in struct, throw, and array funcs

* libguile/struct.c (scm_make_struct_layout, scm_struct_init)
  (scm_struct_vtable_p, scm_struct_ref, scm_struct_set_x): use string
  and symbol accessors and avoid unpacking strings and symbols

* libguile/throw.c (scm_ithrow): allow wide symbols in the error message

* libguile/unif.c (scm_enclose_array, scm_istr2bve): use string
  accessors and avoid unpacking strings