Improve projected scalars cache lookup

(Based on https://github.com/NethermindEth/zinc-plus/commit/6517386a764d71f6b5e2cc3dbd2971a28a1c8321)

Projected scalar cache is currently keyed by `U::Scalar` - so `DensePolynomial<R, 32>`. Hashing it is somewhat expensive and hashmap lookups need to be done repeatedly. Investigate opportunities to avoid hashing it.