random forest: updated test protocol

summary:

• implements the new test protocol for random forest
• uses treelite → put random-forest.so in the code dir
• gets device id from .env file
• misc:
  • moved some existing functions to utils.py
  • made some tests, using onnx
• made it so that witty_record() can return the return value of the measured function — this saves some execution time

cd code
pip3 install -r requirements.txt
echo "DEVICE_ID=your-personal-device-id" > .env
python3 main.py --run-id 0