Traversal to choice element

[johngrimes]

Currently, we only support traversal to "resolved virtual elements" within any-types, e.g.:

Observation.valueQuantity

It would be good if we could also support traversal to the choice element, and then use the ofType function to resolve this to a typed element, e.g.:

Observation.value.ofType(Quantity)

This makes the implementation technically valid with respect to the specification.