Add WS_Builder benchmarks

[matthewfeickert]

As pointed out in Issue #77, @vincecr0ft has made WS_Builder benchmarks at WorkSpaceBuilder. These need to be integrated into the pyhf benchmarking suite.

First thoughts on steps:

• [ ] Understand benchmarks
• [ ] Reimplement without ROOT (Vince's feedback will be helpful here)
• [ ] Move into pyhf's pytest framework

[lukasheinrich]

what we need for this is e.g. a setup where we can generate a) JSON specs and b) XML + ROOT files from a single source of truth

           source of truth (could just be the JSON spec, either from disk or generating a bunch on-the fly)
             /           \
     XML + ROOT   JSON spec (if needed)
            |            |
        ROOT HF         pyhf
            |            |
      results.json      results.json
             \           /
       validation by comparison
              benchmarking

[matthewfeickert]

Given this I should probably complete Issue #3 first.

[matthewfeickert]

For future reference, a solution to the benchmark naming problems I was running into with pytest-benchmark is to simply pass the --becnhmark-histogram option last.