Add first prototype of string with stack allocation.

tests/test-mstring.c

@@ -455,7 +455,7 @@ static void test0(void)
   string_set_str (s1, "");
   assert (string_capacity(s1) > 0);
   string_reserve (s1, 0);
-  assert (string_capacity(s1) == 0);
+  assert (string_capacity(s1) == sizeof (str_heap_t) - 1);
   
   string_set_str(s1, "HELLO XXX!");
   string_replace_at(s1, 6, 3, "World");