Use level-saving plaintext ciphertext multiplication

I was looking at poly approximation lowerings. By default, they use chebyshev paterson stockmeyer with degree 5 polynomials, which should end up using a computation with 3 multiplicative depth. But the lowering consumes multiplicative depth 6 because of the plaintext coefficient multiplications.

Instead, you could use a level-saving trick by encoding the plaintext with the auxiliary modulus P and then rescale without consuming a q in the chain. You can do this in Lattigo, but I'm not totally sure how to configure this in the mgmt passes.

This issue will investigate the possibility of doing this.

Start by reading the poly approximation lowerings and the management passes that configure Chebyshev–Paterson–Stockmeyer polynomials. Compare how plaintext coefficient multiplications are represented with the level-saving approach described for Lattigo. Done means establishing whether the lowering can avoid the extra multiplicative depth and documenting or implementing the required pass configuration.

Written by the indexing model from the issue text.