Carp

For the hardened builds in Carp and we don't want to have unbounded undefined behavior, we want to put hard limits on that. For arithmetic overflow, after lots of thinking about exactly what the right semantics are, we ended up with a model that's somewhat like C++, but also somewhat like Rust. We're still experimenting there to decide, from a performance standpoint, can we reasonably say, oh, it's always defined, but you just get back some unspecified value?