Pick a benchmarking system

[franziskuskiefer]

• https://github.com/google/benchmark
• https://github.com/polubelova/hacl-star-benchmarks

[franziskuskiefer]

Google benchmarks

We'll go with google benchmarks, fitting nicely with the gtests we use already.
They don't count cycles but should be good enough to get quick responses and ensure that we are not regressing performance.

---------------------------------------------------------------------
Benchmark                           Time             CPU   Iterations
---------------------------------------------------------------------
BM_Chacha20_32_encrypt           1553 ns         1553 ns       427520
BM_Chacha20_Vec128_encrypt        824 ns          824 ns       851260

They also allow for easy comparison of benchmarks and hence finding regressions.

Comparing chacha_bench.json to build/Release/chacha20
Benchmark                                    Time             CPU      Time Old      Time New       CPU Old       CPU New
-------------------------------------------------------------------------------------------------------------------------
BM_Chacha20_32_encrypt                    +0.0223         +0.0222          1519          1553          1519          1553
BM_Chacha20_Vec128_encrypt                +0.0166         +0.0166           811           824           811           824
OVERALL_GEOMEAN                           +0.0192         +0.0189             0             0             0             0

#74 sets up the basics but needs cleanup before merging.

[franziskuskiefer]

Done in #74