Generalize the expression framework with two new type variables

[dannywillems]

I think it will be worth to generalize the expression framework over two other types:

• Fiat Shamir Challenges
• Constants

We wil need it later, I think.
We can then use the same expression framework to define the folding expressions.

An expression has four type parameters, at least:
'f, 'challenges, 'constants, 'columns

'f -> the field the values will be defined over
'challenges -> Fiat Shamir challenges -> make it generic to go from any polynomial IP to non interactive polynomial proofs.
'constants -> to make generic over the curves, the permutation we might use, etc
'columns -> the polynomial variables