Implement a variant of proposal #1 for stateful apply in Myrial

This PR adds support for stateful apply by adding a USING clause to the existing bag comprehension.  For example:

```
out = [FROM X EMIT _sum/_count AS _avg USING  0: _count + 1 AS _count, 0: salary + _sum AS _sum];
```

I like this better than the proposal because it makes it clear that the state variables are not embedded in a particular column expression.  Rather, the state variables are free-floating and can be accessed from any column.  This approach also happens to be easier to implement.

comments?