Monomials with ScaledSum

[naure]

There is this hack in the MockProver

let expr = expr.clone().neg().neg(); // TODO get_ext_field_vec doesn't work without this

which only works if the expression happens to use ScaledSum as produced by the * operator. So use that instead of Product.

But then is_monomial_form won’t recognize the zeros in ScaledSum, so add this support too.

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Meanwhile, I chased the cause of this through this stack but I don’t know how to fix it:

• The hack: run_maybe_challenge

• Code that wants an Ext not a Base field: get_ext_field_vec

• Code that produces Ext or Base depending on complicated logic: wit_infer_by_expr

[naure]

Already included in another PR.