Affine y-coordinate access

[nickray]

Understanding the arguments pro/con coordinates from #817...

For a generic-ish Frost implementation, I have need to determine if the output of distributed keygen has even y-coordinate, to perform the Taproot adjustment. I can get away with a local AffineYCoordinateParity trait (fn has_even_y(&self) -> bool) that I implement on k256 and bound on, but maybe "for symmetry" it wouldn't be that horrible to add either AffineYCoordinate or at least AffineYCoordinateParity to the AffineArithmetic::AffinePoint bounds?

[tarcieri]

See this PR for some additional context: RustCrypto/elliptic-curves#50

[tarcieri]

Some discussion of solving this in the group crate: zkcrypto/group#30

[tarcieri]

#1237 has the best description of the current state of affairs and the evolution of these traits