Allow checking for overflow without penalising fast path

[unzvfu]

Currently, to assert that addition doesn't overflow, we

add_cy(s, cy, a, b);
assert(digit::is_zero(cy));

If NDEBUG is not set, then this checks that overflow hasn't occurred. If NDEBUG is set, it's possible (need to verify) that some carry handling code is generated; this is wasteful as it isn't used.

[unzvfu]

Follow up at unzvfu#31.