HashToPoint should re-hash at every iteration, instead of incrementing

[HarryR]

For example, the from_y_always is agnostic to the hashing algorithm which is first used to derive a field element. However, it does this by incrementing the coordinate until a point is valid.

See: https://github.com/HarryR/ethsnarks/blob/master/src/jubjub/point.cpp#L96

This should be changed to:

while True:
  y = H(y)
  x = derive_x(y)
  if x is not None:
    return x, y

[HarryR]

This will be a breaking change, and will invalidate many existing test vectors.