replace itrunc(Uint8, 1234) => mod(1234, Uint8), etc. [fix #8646]

Here's the proposal I put forth in #8646 to replace `itrunc(Uint8, 1234)` etc. with `mod(1234, Uint8)`. Not necessarily how we want to do this, but at least it (hopefully) identifies all the places in base where this needs to be changed.
ah, yes. it was initially unclear to me if that was *this* kind of `itrunc`, but it must be.