`Segment` now includes a `Type` of struct kind. This is intended to
emit the segment as a struct later in decompilation. The implementation
of name has also been added.
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.