Remove "using namespace std" from library/longnum_engine.cpp.
Fix "longnum_print" to use format "%hhd" instead of "%d".
Use sum_tmp to keep in-digit sum result for "longnum_multinum_sum".

At first, I think char space in struct itself is enough to keep it. But it turns out that, in some extream case, overflow can be hapened. This fix "longnum_multinum_sum" to be able to handle more longnum.