Support u256 datatype at HIR level

[DavePearce]

This is the first step towards field agnosticity. Specifically, we introduce a u256 type which compiles into two u128 limbs for the BLS12-377 curve. This would only have an effect when the type was used, meaning that the existing constraints could be compiled as is. However, we would then be able to incrementally update modules.

At the HIR level we can support this type directly by evaluating using big.Int rather than fr.Element. We can presumably add additional checks for overflow, etc as well.