Add machine learning functions plugin

Add functions to train and use machine learning models (classifiers and
regressors) in Presto. This is currently only a proof of concept, and is
not ready for use in production. Example usage is as follows:

SELECT evaluate_classifier_predictions(label, classify(features, model))
FROM (
    SELECT learn_classifier(label, features) AS model
    FROM my_training_data;
)
CROSS JOIN my_validation_data;