Witness multiplication optimization

[ryanleh]

Description

Currently, the witness polynomial multiplication in the second round of the AHP requires performing an FFT with a domain of size 4 * |H|. This change distributes the multiplication so that it only requires an FFT with a domain of size 2 * |H| along with some cheap multiplications/additions.

Running marlin-benches on a fairly powerful machine from master:

per-constraint proving time for Bls12_381: 51662 ns/constraint
per-constraint proving time for MNT4_298: 45649 ns/constraint
per-constraint proving time for MNT6_298: 55951 ns/constraint
per-constraint proving time for MNT4_753: 321626 ns/constraint

Running marlin-benches on the same machine from this PR:

per-constraint proving time for Bls12_381: 50834 ns/constraint
per-constraint proving time for MNT4_298: 46081 ns/constraint
per-constraint proving time for MNT6_298: 54172 ns/constraint
per-constraint proving time for MNT4_753: 320387 ns/constraint

So it seems to give an overall slight latency improvement and reduces the memory overhead of proving. I would guess that performance gains would be more noticeable on weaker machines.

This PR depends on https://github.com/arkworks-rs/algebra/pull/258

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