convert wuffs asserts into back-end asserts

Can there be an option to convert wuffs-side asserts into asserts in the generated code?

This could be a macro which lets the user code pick which behavior to have for them.

So the assert can translate into nothing (like it is now), or a full error like `if not cond {return "#internal error: failed assert";}`, or an actual `assert(cond);`, or even an `assume(cond);`.

Theoretically the compiler should be able to optimize out all these checks but there will be cases where it cannot make the assumptions necessary like across a yield. This in turn could help the optimizer figure out better options based on the invariants that are present in wuffs but not present in the backend.