Jubjub stdlib

[alvaro-alonso]

@dark64 following our last conversation on jubub, I managed to give another look over the holidays and found a way to make nonStrictUnpack256 to adjust the padding dynamically to the FIELD_SIZE_IN_BITS while keeping the original interface of the method, reducing the risk of incompatibilites and having much less files changed.

Can you give it a look a provide some feedback? Thanks :)