[challenge] Bounded types

[Adam D. Ruppe]

> Your code is wrong in many places. In short, exceptional situation should
leave your object in correct state.

Yeah, I know, but I wanted to get the overflow flag check working first and worry
about transactional integrity later.

The way I was thinking of doing it is storing the result in a temporary, which
gets the checks, then having scope(success) commit it to the _payload.

But I just haven't gotten around to it yet - it's just a starting point, not a
finished product.