ToCoding() on ICoded does not work well with choice types

[ewoutkramer]

The interface ICoded allows users to retrieve the "default codes" from a resource instance, these are the coded values from some bound element in a resource that is designated to be "the" code that represents the (category) of that instance. This functionality is used by the CQL Retrieve statements to filter resources by their default coded property.

Some of these properties are choice properties, for which a choice might not represent a bound element. At this moment our SDK throws an exception, but since this case is quite common, it is better to not return and codes from ICoded.ToCodes when a choice element contains a data value that is not bindeable.

While we're at it, we don't have support here for the new R5 CodeableReference, so we should add that too.