Add SWIFFT performance testing

[yarongvili1]

I created a cmake build configuration along with a performance tester of the binary 16-bit SWIFFT function of this repository as part of an effort to compare this function's performance to the SWIFFT function of LibSWIFFT.

I needed to make code changes, which I put in a new swifftperf branch, to get the tester to build because I found no build scripts nor build documentation in the original master branch. I made no effort to build the entire repository not to clean up the code or compilation warnings.

The tester works by running r rounds of the binary 16-bit SWIFFT function of K2SN-MSS single-threaded and measuring the following:

• Number of cycles per round (cpr): This is the total number of cycles taken by all r rounds divided by r.
• Number of cycles per byte (cpb): This is cpr divided by 128, the number of input bytes in each round with the K2SN-MSS SWIFFT parameters n=64 and m=16.