Perf-timer

Fixed
- employ `atexit()` to robustly log results even when `__del__` finalizers are
not called

Added
- perf-timer classes now support tracking various statistics
including standard deviation and percentiles. The options are
`AverageObserver`, `StdDevObserver` (default), and `HistogramObserver`.
E.g. `PerfTimer(..., observer=HistogramObserver)`.
- Benchmark overhead of the various observer and timer types

Fixed
- Support rename of trio.hazmat to trio.lowlevel
- Expose docs to help()

Initial version