profiling: make profiling functions context managers (API) This makes profiling more flexible since we can now call multiple functions when a profiler is active. But the real reason for this is to enable a future consumer to profile a function that returns a generator. We can't do this from the profiling function itself because functions can either be generators or have return values: they can't be both. So therefore it isn't possible to have a generic profiling function that can both consume and re-emit a generator and return a value.