Fix witgen for bus accumulators

[georgwiese]

Fixes #1604

With this PR, we bypass machine detection during witness generation of stages > 0. See this comment for a motivation.

This currently needs to be tested manually, as follows:

$ RUST_LOG=trace cargo run pil test_data/asm/block_to_block_with_bus.asm -o output -f --field bn254 --prove-with halo2-mock
...
===== Summary for row 7:
    main.acc1 = 20713437912485111384541749944547180564950035591542371144095269313127123163196
    main.acc2 = 5162472027861336027760332823162682203738251621730423286600997430635718406729
    main.z = 3
    main.res = 9
    main_arith.acc1 = 1174804959354163837704655800710094523598328808873663199602934873448685332421
    main_arith.acc2 = 16725770843977939194486072922094592884810112778685611057097206755940090088888
    main_arith.acc1_next = 463668501342879563405020640323131794083013726819708055681247370540753473777
    main_arith.acc2_next = 20043340305711842349747334022818855888193664738087810292789887394167185113571
    main_arith.y = 1
    main_arith.z = 1
    main.dummy = 0
    main.acc1_next = 0
    main.acc2_next = 0
    main_arith.x = 0
    main_arith.sel[0] = 0
---------------------
...

Computing main.acc1 + main_arith.acc1 and main.acc2 + main_arith.acc2 both yields 21888242871839275222246405745257275088548364400416034343698204186575808495617, which is the BN254 scalar field prime! In other words, the partial accumulators sum to 0.

[georgwiese]

These (and the changes below) were bugs.

[leonardoalt]

open issue?

[georgwiese]

This needs to be done all over the place (we have the same TODO many times) and is tracked in #1499.