* libguile/strings.c (scm_string, scm_c_make_string):
* libguile/srfi-13.c (scm_reverse_list_to_string, scm_string_map)
  (scm_string_unfold, scm_string_unfold_right, scm_xsubstring)
  (scm_string_filter, scm_string_delete): Remove
  scm_i_string_start_writing / scm_i_string_stop_writing calls around
  fresh strings that aren't visible to other threads.