Signed sliding windows for EdDSA

Signed sliding windows for EdDSA

Signed sliding windows are effectively one bit wider than their unsigned
counterparts, without doubling the size of the corresponding look up
table.  Going from 4-bit unsigned to 5-bit signed allowed us to gain
almost 17 additions on average.

This gain is less impressive than it sounds: the whole operation still
costs 254 doublings and 56 additions, and going signed made window
construction and look up a bit slower.  Overall, we barely gained 2.5%.

We could gain a bit more speed still by precomputing the look up table
for the base point, but the gains would be similar, and the costs in
code size and complexity would be even bigger.