Properly constrain u32 values

[plafer]

As discussed in #1480 (review), AIR constraints currently don't verify that u32 values are $< 2^{32}$. We should enforce that in the constraints.

[bobbinth]

I wouldn't probably call this a bug - more like a sub-optimal design choice :)

Btw, to make sure that all inputs are u32 values, we'd need perform additional range checks (2 range checks per operand). This shouldn't be too bad once we have LogUp-GKR integrated. But we'll also need to add more helper registers - and I'm not yet sure how to do this efficiently.