contrib: add a write microbenchmark to perf.py I'm adding some performance logging to ui.write - this benchmark lets us confirm that the cost of that logging is acceptably low. At this point, the microbenchmark on Linux over SSH shows: ! wall 3.213560 comb 0.410000 user 0.350000 sys 0.060000 (best of 4) while on the Mac locally, it shows: ! wall 0.342325 comb 0.180000 user 0.110000 sys 0.070000 (best of 20)