Improve projected scalars cache lookup

[frozenspider]

(Based on 6517386)

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.

[frozenspider]

I don't see any performance with 6517386 when rebased on top of current main (ff64869), I also tried different caching approach, so closing this for now.
If needed, cache introduction would be simplified by #199