Benchmark support

[skogsbaer]

It should be possible to define benchmarks in HTF. Benchmark should use criterion for measurements.

Possibility 1:

bench_fact = nf fact 100

This simply runs the benchmark when explicitly requested but not when executing the whole test suite.

Possibility 2:

bench_fact = withReference someReferenceBenchmark 3.4 $ nf fact 100

This checks that running nf fact 100 is at most 3.4 times as slow as `someReferenceBenchmark. This benchmark is run when the whole test suite is run.

Benchmark should not be executed concurrently to other tests.

[nikita-volkov]

+1, This would be amazingly useful to have! Great idea!