Query result iterator should return objects

[malthe]

The initial API design was more or less compatible with https://node-postgres.com/, i.e. pg.

In particular, query results were composed of rows and names (although the pg library provides fields from which you can get to the column names).

We want to preserve this result interface, while also providing a more ergonomic and idiomatic interface which we will call the iterator interface:

const result = client.query<{ message: string; }>(
    "SELECT 'Hello ' || $1 || '!' AS message",
    ['world']
);

for (const item of await result) {
    // 'Hello world!'
    console.log(item.message);
}

The awaited result object still provides the rows and names properties (as well as other properties, currently status which is the protocol statement status such as "SELECT 1" indicating that one row was selected).

In addition, rows will now be typed such that we have row.get(name: keyof T) returning a value which matches the generic type parameter.

This issue was spawned from the discussions in #66.