Add machinery for "assert in debug mode" checks

[zbanks]

Some kind of assert-only-in-debug-mode would be useful here. If it comes up we'd want to know about it since it's a probable bug, but at the same time it's a complex invariant so it's hard to be 100% sure about a hard assert, and there's an easy fallback path. Maybe too much effort to be worth introducing machinery for, though.

Originally posted by @simonlindholm in #144 (comment)