Generalize multiplicity witgen

[georgwiese]

(mostly cherry-picked from #1958)

This PR generalizes the multiplicity witness generation we had in FixedLookup in the following ways:

• Instead of searching for a column of a specific name, takes the witness column from the phantom lookups
• As a result, it is also able to handle arbitrarily many lookups
• The witness generation should work for all machine types which can be connected via lookups: Fixed lookup, block machines, and secondary VMs.

A common use-case that is not yet covered is global range constraints, as they are not represented as machine calls in witness generation. This should be fixed in a follow-up PR.

[georgwiese]

Nice, adding the multiplicity to Connection instead of maintaining another map improved the code a lot I think! Now, the code to create the fixed lookup is outrageously verbose and contains many duplications, planning to fix that before the next round of review.

[georgwiese]

OK, I think this is ready for another look :)