benchmark for rotate_cpx_vector

This commit introduces google benchmark library and one benchmark for rotate_cpx_vector.

This is an example of how to write new benchmarking code.

To properly compare results of a benchmark one must first ensure their system is configured for benchmarking.

1. Set cpu governor to performance

sudo cpupower frequency-set --governor performance

2. disable boosting (reduces variance):

https://www.kernel.org/doc/Documentation/cpu-freq/boost.txt

3. Use cpuset to isolate cpus you will be running on (reduces interrupts)

See https://llvm.org/docs/Benchmarking.html#linux, https://man7.org/linux/man-pages/man7/cpuset.7.html

This should give more reproducible results.