Remove `ISystemAndCode` in favor of `ICoded`

[ewoutkramer]

ISystemAndCode is only used to get the system+code out of a Code<T>. There is a less direct route already available via ICoded, which will work on any coded type to get a Fhir.Coding type, which has a system+code.

• Extend this route by also having all bindable datatypes implement ICoding, which will make the current extension method to get a coding out of a DataType unnecessary. Make sure the generated Resource.ICoding implementation is calling this interface, instead of the extension method. Remove the extension method.
• Remove ISystemAndCode, this is a breaking change.