Merge #741: Remove unnecessary sign variable from wnaf_const

Merge #741: Remove unnecessary sign variable from wnaf_const

37dba329c6cb0f7a4228a11dc26aa3a342a3a5d0 Remove unnecessary sign variable from wnaf_const (Jonas Nick)
6bb0b77e158fc2f9e56e4b65b08bcb660d4c588b Fix test_constant_wnaf for -1 and add a test for it. (Jonas Nick)

Pull request description:

  There currently is a single branch in the `ecmul_const` function that is not being exercised by the tests. This branch is unreachable and therefore I'm suggesting to remove it.

  For your convenience the paper the wnaf algorithm can be found [here (The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar Multiplications Secure against Side Channel Attacks)](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.563.1267&rep=rep1&type=pdf). Similarly, unless I'm missing something important, I don't see how their algorithm needs to consider `sign(u[i-1])` unless `d` can be negative - which doesn't make much sense to me either.

ACKs for top commit:
  real-or-random:
    ACK 37dba329c6cb0f7a4228a11dc26aa3a342a3a5d0 I verified the correctness of the change and claimed invariant by manual inspection. I tested the code, both with 32bit and 64bit scalars.

Tree-SHA512: 9db45f76bd881d00a81923b6d2ae1c3e0f49a82a5d55347f01e1ce4e924d9a3bf55483a0697f25039c327e33edca6796ba3205c068d9f2f99aa5d655e46b15be