Add JMH Benchmarks for the critical code paths

[aepfli]

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Our tool is excellent, but more and more people are using it. We already received internal information, that some code paths tend to be hotspots, especially structure operations.

Evaluating this without a proper test harness around it is hard; hence, it might be a good idea to add a JMH Suite to our tests for verification and to allow us to spot hotspots earlier in the future.

• https://github.com/openjdk/jmh
• https://www.baeldung.com/java-microbenchmark-harness