ACIR + WitnessMap structures codegen as build step output

[kobyhallx]

CodeGen step needs to be introduced to have cpp code generated from ACIR and WitnessMap structs.

[TomAFrench]

This would require either a breaking change to rename brillig::Opcode to be distinct from acir::circuit::Opcode or we'd need to perform some automated renaming step before we can generate this.

This breaking change is the reason why we haven't merged this logic into master currently.