Name the field for string backing store

* libguile/strings.h (chars): Name the field.
* libguile/strings.c (string_stringbuf, string_aliased_string)
(make_string, scm_i_string_ensure_mutable_x, scm_i_substring_shared)
(scm_i_try_narrow_string, scm_i_string_set_x): Adapt.

libguile/strings.h

@@ -202,7 +202,7 @@ struct scm_string
     struct scm_stringbuf *stringbuf;
     /* Sometimes though it aliases another string.  */
     struct scm_string *string;
-  };
+  } chars;
   size_t start;
   size_t length;
 };