Make collection iterable using for of loops

[fl0w]

• Related Issues: Make Collections iterable #1816, iterate collection (for of) #1525
• Previous PRs: Make Collection iterable #1817

Introduction

Make collections iterable using for of loops.

const models = await bookshelf.model('Model').fetchAll()
// previously
for (const model of models.models) {}

// now
for (const model of models) {}

Motivation

It's nice to be able to use default ECMA functionality when they're non-breaking.

Proposed solution

Define CollectionBase.prototype[Symbol.iterator] and yield to this.models (which always defaults to an array)

Current PR Issues

None that I am aware of

Alternatives considered

None that I am aware of

[fl0w]

My god, I completely missed #1817, still think my solution is cleaner. Apologies :)

[ricardograca]

This needs to have accompanying tests to ensure the correct behavior.

[fl0w]

Added a simple test, if it's not enough pointers are appreciated and I'll add tests accordantly.