Implement needed distance-bounding protocol in library

[dbosk]

We need to perform the computations needed in the distance-bounding protocol, i.e. those computations done in the setup and verification phases. It seems to be difficult to implement the distance-bounding phase other than in hardware. But maybe Sébastien has more insights.

[dbosk]

As of now we will implement the DB Schnorr protocol (Sect. 6 in the paper). This will take enough of the distance bounding into consideration. We only need a hardware chip where we can load the strings $s_0$ and $s_1$ to complete it.

This is now merged with #54 .