Add benchmarks

[Woyten]

The current performance does not seem to be a a problem. microwave can render 800 Sin stages and 1600 Sawtooth stages simultaneously on a single Core i7-6700HQ CPU @ 2.60GHz. There is still room for improvement by rendering the Sin function to a wavetable (#28) and by using all CPU cores.

However, thinking about performance without measuring it is a problem. Therefore, I would like to add some benchmark tests that confirm or disprove performance considerations.

Vision:

• Add a bench subcommand which runs some performance test (e.g. measures the time to render 100.000 Sin stages into a buffer)
• Create an independent github action which will build the microwave executable for all commits since the latest release tag in --release mode
• Run microwave bench for all builds multiple times and create a benchmark report

[Woyten]

Added with 84d8c75.