Refactor: Marshal

[brandonc]

I reorganized the very large visitModelNode function (the internal logic that builds a Node struct by recursively descending into a jsonapi-annotated struct model) into a couple of abstractions:

1. modelFieldCursor encapsulates reflection data about each field in a model struct.
2. modelVisitor uses cursor method Next() to move the cursor to each field. It retains config about sideloading relations as included Nodes and provides the top level Visit() interface. Most of the field-specific code was relocated as methods of the visitor, each receives the cursor as an argument.

[brandonc]

I don't think this is ready because I just benchmarked it and it appears to be slower than the main branch. I'll keep working on it.