This check-in provides the fix for leaks in ECP's Add() and Double(). The fixes were taken from Joost Renes, Craig Costello, and Lejla Batina's [Complete addition formulas for prime order elliptic curves](https://eprint.iacr.org/2015/1060.pdf).
The Pull Request includes two additional changes that were related to testing the primary fix. First, an `AuthenticatedKeyAgreementWithRolesValidate` interface was added. It allows us to test key agreement when roles are involved. Roles are "client", "server", "initiator", "recipient", etc.
Second, `SetGlobalSeed` was added to `test.cpp` to help with reproducible results. We had code in two different places that set the seed value for the random number generator. But it was sloppy and doing a poor job since results could not be reproduced under some circumstances.
Thanks to Jack Lloyd and Botan for allowing us to use the implementation.
The numbers for SSE2 are very good. When compared with Salsa20 ASM the results are:
* Salsa20 2.55 cpb; ChaCha/20 2.90 cpb
* Salsa20/12 1.61 cpb; ChaCha/12 1.90 cpb
* Salsa20/8 1.34 cpb; ChaCha/8 1.5 cpb
