Add function _parse_str for string.

tests/test-mstring.c

@@ -288,6 +288,16 @@ static void test0(void)
   string_get_str(s2, s1, true);
   assert (string_cmp_str (s2, "\"Hello world\"\"Hello \\\"world\\\"\"") == 0);
 
+  const char *sp = string_get_cstr(s2);
+  b = string_parse_str(s1, &sp);
+  assert (b == true);
+  assert (strcmp(sp, "\"Hello \\\"world\\\"\"") == 0);
+  assert (string_cmp_str(s1, "Hello world") == 0);
+  b = string_parse_str(s1, &sp);
+  assert (b == true);
+  assert (strcmp(sp, "") == 0);
+  assert (string_cmp_str(s1, "Hello \"world\"") == 0);
+  
   string_set_str(s1, "Hello \"world\"");
   FILE *f = fopen ("a-mstring.dat", "wt");
   assert (f != NULL);