ml-kem: Don't use FieldElement for compressed values

[bifurcation]

FieldElement should specifically represent a member of $Z_q$ (can't figure out how to get GitHub to do \mathbb). Right now, it is also used as the output of the compress() method as well, even though these are elements of $Z_{2^d}$. We should define a separate type for this latter type of number, something like CompressedValue<D: CompressionFactor>