ml-kem: potential timing vside channel converting message to polynomial

[bwesterb]

You might want a preemptive fix for a sidechannel converting message to polynomial.

It might not affect Rust at the moment.

A fix could look like this.

[tarcieri]

Dup of #25

[tarcieri]

@bwesterb FWIW, after shipping a similar fix in curve25519-dalek which used an inline black_box function which also does a volatile read and being a bit unhappy with that as a solution, I convinced upstream to reword the documentation around core::hint::black_box as it seems like a better (although previously scarily documented) solution.

We also just added a BlackBox type to subtle which requires all reads go through black_box which might be useful for these sorts of applications.

[bwesterb]

Sounds good. Sorry about the dup — missed it while skimming the list of issues 🤦.