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.