Make field alias optional.

### Overview

Changes the `SELECT` fields to not require an alias so it's more like SQL. If no alias is specified then the alias becomes an underscored version of a concatenation of the aggregation name and (optionally) the expression. Any continuous run of non-alpha characters in the expression are converted to a single underscore.


### Examples

The following queries are equivalent:

```skyql
SELECT count() AS count
SELECT count
```

```skyql
SELECT sum(unit_price) AS sum_unit_price
SELECT sum(unit_price)
```

```skyql
SELECT avg(price * 100) AS avg_price
SELECT avg(price * 100)
```

```skyql
SELECT max((quantity * price) / 100) AS max_quantity_price
SELECT max((quantity * price) / 100)
```