Fix Edwards curve coordinate naming conventions in the code

[str4d]

In the original Jubjub implementation from sapling-crypto, the coordinates of affine Edwards points were labelled (x, y). However, the jubjub crate (which we switched to in #272) labels them as (u, v), as does the protocol spec. This has resulted in the codebase mixing conventions. We should clean up the codebase to refer consistently to (u, v).