Make Result iterable

[vitaly-t]

Make Result automatically iterable, so one can iterate through rows without having to access rows explicitly:

client.query('select...', (err, res) => {
    // below we can use just "res", not "res.rows"
    for(const a of res) {
        console.log(a); // print the row
    }
})

This also simplifies data processing when you want to process it as just iterable, all the way, not as an array.

[vitaly-t]

In an afterthought, it would make the library perform better, if rows array is retired, and replaced with an iterator, because it doesn't make sense executing all that data-type conversion unless it is actually used. In many cases, particularly with large data results, the client ends up not ever using the data, rather a small portion of it, like with paging, for example.

This PR above is a first step toward that. For now, it just forwards into rows values, but later it may implement real iteration through data, with dynamic data-type conversion, on-demand.

The bottom line, modern development practices gravitate toward iterables versus pre-built lists.

[vitaly-t]

Added the same change into pg-native, to be merged at the same time.

[charmander]

I don’t think this is worth changing now.

• Multi-statement queries return an array of results that is also iterable, which could be confusing.
• res.rows, {rows}, or a wrapper function are easy to write.
• People will often want to use array methods, which aren’t available on the plain iterable anyway.

It’s nice when there’s just one way to do it.

[vitaly-t]

This caters for all coding styles, without any change in functionality. It also opens a path to changing types conversion, to become dynamic. There are lots of libraries these days that handle iterables well.