Unify conventions around usage of `big.Int`

[fmoletta]

Some functions use big.Int as arguments, while others use *big.Int. The same happens with scope variables, some hints store them as references while others store their value, which can lead to bugs in cross-hint interaction. We should have a unified criteria to avoid any kind of confusions.
I suggest sticking to *big.Int, as this is what all functions and methods in the big package use