store pqcrystals polynomial in secure memory

[falko-strenzke]

No description provided.

[reneme]

Note that the PolynomialVector class is tightly coupled with the Polynomial class in a sense that it can borrow its byte storage (that's what the redundant std::span is for). I.e. also the std::vector in the PolynomialVector should be replaced by secure_vector then.