Compile benchmark directory

[bwignall]

The benchmark directory was not compiling on a fresh checkout. Some of this was because of changes to the Google Caliper dependency, some to Breeze APIs in the "root" directories, some other.

The benchmark directory now compiles cleanly, locally.

[dlwh]

thanks, yeah I stopped running these on the regular. If you wanted to make some fast version of them run in CI that might be a good idea?

(As you can probably tell, Breeze is maintainence-mode with me flirting pressing the "archive this project" button. I haven't written non-trivial scala for 2.5 years.)

[bwignall]

I must admit that I haven't tried running the benchmarks themselves, so I don't have an immediate sense for how to run a fast version. How about the following? I add a CI step in the existing config, to add a benchmark / compile checking step, and I save the updated runs for a separate PR?

[dlwh]

sure that's great. I'm gonna merge this as-is if you want to follow up with more PR's, please lmk!