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)