ed448-goldilocks: 56-byte decaf x-coordinates

[tarcieri]

Decaf x-coordinates are supposed to be serialized as 56-bytes, but we're serializing them as 57-bytes, with a TODO to fix this:

https://github.com/RustCrypto/elliptic-curves/blob/c797d60/ed448-goldilocks/src/decaf/affine.rs#L63-L68

This has turned into a blocker for this PR to expose AffineCoordinates::y, since that trait assumes both the x and y-coordinates have the same size:

RustCrypto/traits#1891

I think for now I am just going to comment out the AffineCoordinates and CurveArithmetic trait impls for Decaf, until such a time as that TODO can actually be addressed.