Allow out-of-crate preprocessed column definitions

[yoichi-nexus]

This PR removes an enum PreprocessedColumn and introduces a trait PreprocessedColumnOps. The enum values are turned into trait implementations.

Motivation

Before this PR, when a stwo user wanted a new column in the preprocessed column, they had to change stwo codebase and add a value in PreprocessedColumn enum.

After this PR, they can implement PreprocessedColumnOps in their own crate, without touching stwo codebase.

[reviewable-StarkWare]

This change is 

[yoichi-nexus]

I got early feedbacks. I want to give it another try

• I'll keep the new trait as is
• I'll add type PreprocessedColumn: Preprocessed ColumnOps in the EvalAtRow interface

This would eliminate the dynamic dispatch business, and will be a straightforward generalization of the current codebase.

[yoichi-nexus]

I'll add type PreprocessedColumn: Preprocessed ColumnOps

Didn't quite work out. I am adding tons of type arguments everywhere. At the moment this PR is still my best try.

[ilyalesokhin-starkware]

crates/prover/src/constraint_framework/preprocessed_columns.rs line 27 at r1 (raw file):

    fn gen_preprocessed_column_simd(
        &self,
    ) -> CircleEvaluation<SimdBackend, BaseField, BitReversedOrder>;

are we going to have a function for every backend?

Code quote:

    fn gen_preprocessed_column_cpu(
        &self,
    ) -> CircleEvaluation<CpuBackend, BaseField, BitReversedOrder>;
    fn gen_preprocessed_column_simd(
        &self,
    ) -> CircleEvaluation<SimdBackend, BaseField, BitReversedOrder>;

[ilyalesokhin-starkware]

Reviewed 3 of 14 files at r1, all commit messages.
Reviewable status: 3 of 14 files reviewed, 1 unresolved discussion (waiting on @yoichi-nexus)

[yoichi-nexus]

are we going to have a function for every backend?

Yes, that will happen in this approach.

Passing <B: Backend> here is not compatible with dynamic trait objects dyn PreprocessedColumnOps. So the approximation is a function for each backend.

Alternatives:

• a single function that takes enum Backend
• a single function that takes dyn Backend
• giving up dyn PreprocessedColumnOps and adding a type argument P: PreprocessedColumnOps to EvalAtRow
• giving up PreprocessedColumn and letting users prepare their column vectors on their own
• keeping the fixed menu of preprocessed columns