[Minor] Reduce duplication in the Cython/C++ for sandwich and dot products.

[tbenthompson]

There are several places where there's a lot of duplication: dense.pyx, dense_helpers-tmpl.cpp, categorical.pyx.

I'm not sure how feasible or reasonable it is to reduce this. It's going to be hard to do so without impacting performance.

[ElizabethSantorellaQC]

Closed in favor of Quantco/tabmat#5