Ideally we'd use %zu for size_t args, but that's unlikely to be supported
by all runtimes, and we would then need to implement it in mp_printf.
So simplest and most portable option is to use %u and cast the argument
to uint(=unsigned int).

Note: reason for the change is that UINT_FMT can be %llu (size suitable
for mp_uint_t) which is wider than size_t and prints incorrect results.