Expose y-coordinate in AffineCoordinates too

[ia0]

I'm not sure why trait AffineCoordinates doesn't also have an fn y(&self) -> Self::FieldRepr similarly as for the x-coordinate. In particular given that's how it is stored in struct AffinePoint.

Would it be possible to also add such a method? Possibly as a provided method (and computed from x() and y_is_odd()). The AffinePoint implementation may explicitly implement it to avoid round-tripping.

[tarcieri]

Dup of #1019