Fun: `graphile-export` can simplify some pg-sql2 expressions

We do a number of optimizations in graphile-export... I just noticed another that we could do would be compressing SQL expressions. For example:

```ts
fragment: sql`${alias}.${sql.identifier("type")}`
```

could easily become:

```ts
fragment: sql`${alias}."type"`
```

or if we're being _really_ fancy:

```ts
fragment: sql`${alias}.type`
```

Wouldn't make a huge difference to performance, but is much easier for humans to read (which is one of the goals of Graphile Export!)