Remove Bench or A generic JHM benchmark interface

[liufengyun]

The old bench/ project is no longer necessary, as it's superseded by:

• https://github.com/liufengyun/bench
• https://github.com/liufengyun/bench-web

Web UI: https://liufengyun.github.io/bench/

We can either remove the project bench completely, or change it in to a generic test interface, as this PR does. This PR will enable developers to run jmh benchmarks in command line:

sbt dotty-bench/jmh:compile
sbt dotty-bench/pack
bench/target/pack/bin/bench  hello.scala    # arguments are the same as compiler options

Note: The separation of benchmarking into a separate repo is justified by the fact that (1) we need to test history of Dotty, thus the test driver has to be independent of versions of Dotty; (2) Dotty doesn't depend on it; (3) faster development process.

[DarkDimius]

Suggestions:

• measure in milliseconds;

• graph multiple lines:

  • average
  • median [50% percentile]
  • 5% percentile
  • 95% percentile
  • 99% percentile
  • trendline for all listed above

• store raw data for runs, so that we don't need to rerun benchmarks.

• add a test that is supposed to always be flat, will be used as indicator of stability of machine.

[DarkDimius]

and please, make the Y axis start from 0.

[DarkDimius]

One more suggestion: could you align the x axis across multiple benchmarks? so that one vertical always relates to the same PR on all benchmarks.

[DarkDimius]

LGTM