Check that serialisation/deserialisation types match [fixes #2242].

The core of the problem is that numeric constant 0 or 1 are of type int. When we serialise AST, we use something like fprinf (out, "foo (1, %zu)", some_sizet_var), which prints to 0, which is int by default. The function definition of foo is something like:

size_t
foo (size_t count, ...) {
   va_start (ap, count);
   size_t x = va_arg (ap, size_t);
   va_end ();
   return x;
}

For more information see #2242 (closed). In this MR at serialisation time we generate not just a numerical value, but also a static assertion that this value is of the right type. The static assertion will terminate compilation of tree files in case the sizes don't match.