UTS tree search - add here or other repo?

[rrnewton]

@mikerainey brought up UTS tree search as a next "parallelism shootout" beyond the well-balanced spawntree/forktree benchmark.

If we make it super cheap to share the per-language setup, then maybe it is ok to make a repo per benchmark. More likely, I think it would make sense to add that benchmark to this same repo and share the build setup.

I.e. can add lang/uts.ext in addition to lang/spawnbench.ext.