Refactor parser and hydrator

[JaZo]

Description

I refactored the Parser and Hydrator into several smaller classes similar to the existing parsers. This should improve the code quality and lower the complexity. Besides that, I made sure everything is properly tested. This is a breaking change if you use one of the parsers yourself instead of the repository.

An important change to note is that relationships with data that is not included in the document are now also hydrated as empty Item.

Given

{
	"data": {
		"type": "foo",
		"id": "1",
		"attributes": {
			"Foo": "bar"
		},
		"relationships": {
			"child": {
				"data": {
					"type": "child",
					"id": "1"
				}
			}
		}
	}
}

Before

$item->child // null

After

$item->child // \Swis\JsonApi\Client\Item

Motivation and context

The Parser and Hydrator are fairly complex and tightly coupled. Besides that, test coverage was poor.

How has this been tested?

Test using new and existing unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[vblinden]

A small inconsistency with the naming of Jsonapi and jsonapi.

We leave it like it is