Partially rewrite JsonApi\Hydrator to improve performance

[JaZo]

Description

I (partially) rewrote the JsonApi\Hydrator to improve the performance.

Motivation and context

Hydrating a document with 2.000 items with relationships took about five minutes. After this change it is reduced to around two seconds. 🎉

Performance was improved by using a 'lookup table' to find the hydrated items instead of looping over all items for every relationship e.g. 2.000 * 2.000.

How has this been tested?

This has been tested/profiled in a private project with timings from laravel-debugbar. Please see this example json.

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:

• 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.

[bbrala]

Awesome! Glad the lookup table worked out so well 👍