Generate literals for enumerable code values

[cmcgee-mac]

The DICOM spec defines what the codes should be for a large number of the tags that hold codes. It might be useful to define Go style types and values for them (See https://stackoverflow.com/a/17989915 and https://golang.org/ref/spec#Iota ).

This is not a small task. It will require crawling more information from the specification.

There are also certain risks. If the type names can be generated using the tag name, like the sequences, and those names are globally unique, then great. It's also possible that the allowed values are different depending on the module. Either these issues will need to be resolved or there will need to be some mitigation balancing the type-safety with the needed expressiveness.