Perfom walltime measurements in nanoseconds

[PhilippWendler]

Since Python 3.7 there is time.monotonic_ns(), which allows us to use nanoseconds for measuring walltime.

This makes sense only if we switch from using float to Decimal for storing these values, as the main reason why time.monotonic() is less precise is the use of float. This will change the types used in the public API of RunExecutor, but we have documented already that these values can be some arithmetic type and we have not guaranteed them to be floats. So we should be able to do this.

Then we should also return CPU-time measurements as Decimal values. (For these we already read micro- or nanosecond values from the kernel, but loose precision by converting them to float.)