Introduce feature to keep AUIPC and LUI

[matthiasgoergens]

At the moment, we are converting AUIPC and LUI to ADDI at decoding time.

We suspect that this will speed up the recursive proof (and have no statistically significant impact on the non-recursive proof).

However we would like to verify that suspicion. Thus this PR asks to introduce a feature toggle keep_auipc_lui, so we can study the impact on recursion performance and proof sizes, once we have recursion working.