Reorganize signature code; make test fail again.

* Reordered functions so that they appear in order similar to
  tweetnacl.c.

* Replaced while loops with for loops to match tweenacl.c.

* In pack25519, change t from empty array to new gf() -- which makes the
  test fail again on 29-byte messages unless we observe n or t with
  console.log. This needs investigation.