WIP: Add machine learning functions

Add functions to train and use machine learning models (classifiers and
regressors) in Presto. This is gated behind the
experimental-syntax-enabled flag, and example usage is as follows:

Train a model and save it to use later:
CREATE TABLE model_table AS SELECT LEARN(label, features) AS model
FROM my_training_data;

Use the model to make some predictions, and evaluate how good they are:
SELECT EVALUATE_CLASSIFIER_PREDICTIONS(label, CLASSIFY(features, model))
FROM model_table CROSS JOIN my_test_data;