Refactor to allow resources to be serialized without the developer needing to understand the internal components

[lgebhardt]

Also simplify and clarify naming for internal components

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Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

This PR primarily reworks the internal components to allow passing a resource, or and array of resources, to the serializer for serialization. This restores a functionality that previously existed in v0.9.x and earlier, but was made very difficult in v0.10.x.

To accomplish this the ResourceFragment is expanded to allow an optional reference to a resource instance. This allows the ResourceTree (renamed from ResourceIdTree) to be constructed from fragments with or without full resources. The ResourceTree now is responsible for loading the included resources instead of the Processor. This allows the serializer to populate the full response (similar to v0.9 and earlier).

Some differences in behavior from v0.9:

• Resources serialized this way do not walk the tree of included models to resolve the includes
• Included resources are retrieved using the two staged flow that's used by the processor (ResourceIdentites are plucked and then resources are instantiated using records_for_populate to fill in missing resources in the full set)