Clean up fj-kernel's iter module

[hannobraun]

The iter module in fj-kernel provides infrastructure to iterate over the objects referenced by another object. This is a critical piece of functionality that is used by the validation infrastructure and other code.

Unfortunately the code in that module is highly redundant. For each object, most method implementations of the ObjectIters look almost exactly the same, because most of those methods just delegate to the corresponding method of the objects being referenced. This results in a lot of code, and a lot of opportunity for subtle errors.

I have two ideas for how to address this, but haven't explored any in depth:

1. Add an additional method to ObjectIters that returns all objects that the implementing object references as dyn ObjectIters. Then the other methods could get default implementations, based on that method.
2. Create a proc macro (maybe even in the form of a derive macro?) that implements the trait for all object types. This is probably overkill, unless idea 1 doesn't work out.