Add jubjub twisted edwards curve to stdlib

[dark64]

#1251

[alvaro-alonso]

@dark64 rebasing the source branch should solve the failing check

[alvaro-alonso]

I keep in mind format and changelog for next time ✍️ everything else looks good to me

[alvaro-alonso]

@Schaeff can you please review the branch? 👾

[dark64]

TODO: After a second look, I figure this function should have better documentation

cc @alvaro-alonso

[alvaro-alonso]

@dark64 good point!

What about:

import "./unpack_unchecked";

// Unpack a field element as 256 big-endian bits.
// Since the size of p can differ among different fields, 
// p_size has to be given to derive the correct padding.
// For example, for bn128 |p|=254, for bls12-381 |p|=255.
// Note: uniqueness of the output is not guaranteed
// For example, `0` can map to `[0, 0, ..., 0]` or to `bits(p)`
def main(field i, u32 p_size) -> bool[256] {
    assert(p_size <= 256);
    u32 padding_size = 256 - p_size;
    bool[p_size] b = unpack_unchecked(i);
    return [...[false; padding_size], ...b];
}

I can not push the changes to the branch. And I think a PR is overkill 💀