Expose stateful apply in Myrial

This PR implements the "Suciu" variant of the stateful apply language.  It strips away the auto-boxing and all extraneous keywords:

```python
APPLY RunningMean(value) {
      [0 AS count, 0 AS sum];
      [count + 1 AS count, sum + value AS sum];
      sum / count;
};
```

Builds on #101.