* 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.
This commit reserves the "unnamed_" prefix for revng.
The first use of this reserved prefix is in `model::Type`s
`CustomName`s and `model::Identifier`s.
This commit also makes the `verify()` method stricter for
`model::Identifier` so that an `Identifier` whose name starts with the
reserved prefix does fails verification.
This commit introduces the type system of the model along with several
various other improvements to the model and its users.
* Introduce the type system.
* Introduce possibility to tag certain fields in the model as to be
optional during YAML serialization.
* All the `Name` fields have been replaced in favor of `CustomName` plus
a `name` method that will use `CustomName` if available, or an
automatically generated name otherwise.
* Make TupleTreeReferences behavior more robust: now you either need to
have a valid pointer to `Root` and a `Path` or be default constructed
(`nullptr` for `Root` and an empty `Path`). Any other configuration is
invalid.
* The type system introduces `RawFunctionType`: this superseds the
previous way in which we were specifying arguments and return
values. Users of such information have been updated accordingly.
Pietro Fezzardi