RedJubJub point serialization

[burdges]

I used ZCash ECC field serialization for points in 05382ea but it should conform with the serializations actually done internally by redjubjub in https://github.com/zkcrypto/jubjub/blob/master/src/lib.rs#L397-L444 Anyone know where this happens in ZCash ECC's code?

[burdges]

I've mostly done this using read and write methods, but ZEXE has no_std methods for this stuff, which sound worth adopting.

[burdges]

Is the current serialization usable @sorpaas ?