RobotPy startup on a RoboRIO is slow

On my RoboRIO with a 2016 image, I get the following (mostly consistent timing between runs, no DS connected):

```
roboRIO-2423-FRC:~$ time python3 -c 'import hal'

real 0m4.954s
user 0m1.280s
sys 0m0.170s
roboRIO-2423-FRC:~$ time python3 -c 'import wpilib'

real 0m6.984s
user 0m3.270s
sys 0m0.260s
```

HAL library loading may be a big chunk of this:

```
roboRIO-2423-FRC:~$ time python3 -c 'import ctypes; ctypes.CDLL("/usr/local/lib/python3.5/site-packages/hal_impl/libHALAthena.so")'

real 0m3.093s
user 0m0.470s
sys 0m0.070s
```

As a baseline:

```
roboRIO-2423-FRC:~$ time python3 -c 'pass'

real 0m0.210s
user 0m0.180s
sys 0m0.020s
roboRIO-2423-FRC:~$ time python3 -c 'import pip'

real 0m3.002s
user 0m2.760s
sys 0m0.220s
```

It would be good to reduce the startup time somehow.