Model classes are now described by a YAML document, which is used to
generate C++ headers containing classes and all the boilerplate
required for YAML serialization/deserialization, usage in
SortedVectors, etc. See the README in include/revng/Model for more
info.
* Introduce some documentation for the model.
* Improve the way enums are serialized/deserialized.
* Mark certain fields of model data structures as optional.
* introduces some error messages during model validation.
Alessandro Di Federico