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.
The previous design returned a model::Binary &, which did not allow
users writing the model to properly initialize cross references between
different parts of the model.
This commit drops the KeyTraits in favor of a std::any-like solution.
Basically, we type erase any key the user wants to employ, just exposing
a virtual version of the destructor, a comparison operator and a clone
primitive.
We used to mark call to noreturn functions as killers, but this is not
correct.
Note that this is a temporary solution, we need to explicitly handle
such situations.
Alessandro Di Federico