Simplify relations code

[codygunton]

Each relation implements a bit of polynomial arithmetic applied to univariate polynomials. As an optimization, we would like that the univariates have the minimum allowable length (the length is the number of evaluations representing the polynomial; this is equals the degree of the polynomial + 1). To accomplish this, each relation defines a container for univariates of different lengths. To avoid code duplication, we have taken a somewhat complicated approach to doing this: each relation is split into

• an implementation, which records these lengths in a template, and
• a uniform 'wrapping'/completion step where this template is used to create the corresponding containers.

This solutions works, but it would be beneficial to simplify this structure. Some possibilities:

• Each RelationImpl puts its lengths in an array which is somehow treated as a parameter pack in order to create the desired containers.
• We accept duplication and just define the containers in each relation. We can enforce that this is done through a concept.

cc @maramihali

[codygunton]

Should have been an issue in Barretenberg repo; see AztecProtocol/barretenberg#720