implement a fix for KOS15

[themighty1]

Our KOS15 implementation is based on the an old KOS15 paper whose proof was recently proven invalid.
The KOS15 paper was updated https://eprint.iacr.org/2015/546 and now contains a fix in Figure 10.

The fix seems straightforward to implement. The paper says
"We have not included various optimizations for reducing communication and computation.". But even in its
unoptimized state, the Protocol in Figure 10 will be performant enough for bootstrapping Ferret OT.

[sinui0]

This will reduce a round of communication too, removing the cointoss protocol.